GEM-STONES
INTERESTING AND IMPORTANT BOOKS
JEWELLERY. By CYRIL DAVENPORT, F.S. A. With a Frontis- piece in Colour and 41 other Illustrations. Seco.nd Edition. Demy i6mo. [Little Books on Art.
JEWELLERY. By H. CLIFFORD SMITH, M.A. With 50 Plates in Collotype, 4 in Colour, and 33 Illustrations in the text. Second Edition. Wide royal 8vo, gilt top.
[Connoisseur's Library.
GOLDSMITHS' AND SILVERSMITHS' WORK. By NELSON DAWSON. With 51 Plates in Collotype, a Frontispiece in Photogravure, and numerous Illustrations in the text. Second Edition. Wide royal 8vo, gilt top.
[Connoisseur's Library.
EUROPEAN ENAMELS. By H. H. CUNYNGHAME, C.B. With 58 Illustrations in Collotype and Half-tone and 4 Plates in Colour. Wide royal 8vo, gilt top.
[Connoisseurs Library.
ENAMELS. By Mrs. NELSON DAWSON. With 33 Illustrations. Second Edition. Demy i6mo. [Little Books on Art.
t'LATK I 'rftttisfiece
-c
V/L'AMAKISE
I. SAI-I'HIRE 12. YELLOW SAI'FHIli
(Oriental Tofaz)
GEM-STONES
GEM-STONES
AND THEIR DISTINCTIVE CHARACTERS
G. F. HERBERT SMITH
M.A., D.Sc.
OF THE BRITISH MUSEUM (NATURAL HISTORY)
WITH MANY DIAGRAMS AND THIRTY-TWO PLATES OF WHICH THREE ARE IN COLOUR
THIRD EDITION
METHUEN & CO. LTD.
36 ESSEX STREET W.G.
LONDON
First Published . . . March lit Second Edition . . . June Third Editum .
PREFACE
IN this edition the opportunity has been taken to correct a few misprints and mistakes that have been discovered in the first, and to alter slightly one or two paragraphs, but otherwise no change has been made. G. F. H. S.
WANDSWORTH COMMON, S.W.
PREFACE TO THE FIRST EDITION
IT has been my endeavour to provide in this book a concise, yet sufficiently complete, account of the physical characters of the mineral species which find service in jewellery, and of the methods available for determining their principal physical constants to enable a reader, even if previously unacquainted with the subject, to have at hand all the information requisite for the sure identification of any cut stone which may be met with. For several reasons I have dealt somewhat more fully with the branches of science closely connected with the properties of crystallized matter than has been customary hitherto in even the most comprehensive books on precious
2005117
vi GEM-STONES
stones. Recent years have witnessed many changes in the jewellery world. Gem-stones are no longer entirely drawn from a few well-marked mineral species, which are, on the whole, easily distinguishable from one another, and it becomes increasingly diffi- cult for even the most experienced eye to recognize a cut stone with unerring certainty. So long as the only confusion lay between precious stones and paste imitations an ordinary file was the solitary piece of apparatus required by the jeweller, but now recourse must be had to more discriminative tests, such as the refractive index or the specific gravity, the de- termination of which calls for a little knowledge and skill. Concurrently, a keener interest is being taken in the scientific aspect of gem-stones by the public at large, who are attracted to them mainly by aesthetic considerations.
While the treatment has been kept as simple as possible, technical expressions, where necessary, have not been avoided, but their meanings have been explained, and it is hoped that their use will not prove stumbling-blocks to the novice. Unfamiliar words of this kind often give a forbidding air to a new subject, but they are used merely to avoid cir- cumlocution, and not, like the incantations of a wizard, to veil the difficulties in still deeper gloom. For actual practical work the pages on the refracto- meter and its use and the method of heavy liquids for the determination of specific gravities, and the tables of physical constants at the end of the book, with occasional reference, in case of doubt, to the descriptions of the several species alone are required ; other methods — such as the prismatic mode of measuring refractive indices, or the hydrostatic way
PREFACE vii
of finding specific gravities — which find a place in the ordinary curriculum of a physics course are described in their special application to gem-stones, but they are not so suitable for workshop practice. Since the scope of the book is confined mainly to the stones as they appear on the market, little has been said about their geological occurrence ; the case of diamond, however, is of exceptional interest and has been more fully treated. The weights stated for the historical diamonds are those usually pub- lished, and are probably in many instances far from correct, but they serve to give an idea of the sizes of the stones ; the English carat is the unit used, and the numbers must be increased by about 2\ per cent, if the weights be expressed in metric carats. The prices quoted for the various species must only be regarded as approximate, since they may change from year to year, or even day to day, according to the state of trade and the whim of fashion.
The diagram on Plate II and most of the crystal drawings were made by me. The remaining draw- ings are the work of Mr. H. H. Penton. He likewise prepared the coloured drawings of cut stones which appear on the three coloured plates, his models, with two exceptions, being selected from the cut specimens .in the Mineral Collection of the British Museum by permission of the Trustees. Unfortunately, the difficulties that still beset the reproduction of pictures in colour have prevented full justice being done to the faithfulness of his brush. I highly appreciate the interest he took in the work, and the care and skill with which it was executed. My thanks are due to the De Beers Consolidated Mines Co. Ltd., and to Sir Henry A. Miers, F.R.S., Principal of the
viii GEM-STONES
University of London, for the illustrations of the Kimberley and Wesselton diamond mines, and of the methods and apparatus employed in breaking up and concentrating the blue ground ; to Messrs. I. J. Asscher & Co. for the use of the photograph of the Cullinan diamond ; to Mr. J. H. Steward for the loan of the block of the refractometer ; and to Mr. H. W. Atkinson for the illustration of the diamond-sorting machine. My colleague, Mr. W. Campbell Smith, B.A., has most kindly read the proof-sheets, and has been of great assistance in many ways. I hope that, thanks to his invaluable help, the errors in the book which may have escaped notice will prove few in number and unimportant in character. To Mr. Edward Hopkins I owe an especial debt of gratitude for his cheerful readiness to assist me in any way in his power. He read both the manuscript and the proof-sheets, and the information with regard to the commercial and practical side of the subject was very largely supplied by him. He also placed at my service a large number of photographs, some of which — for instance, those illustrating the cutting of stones — he had specially taken for me, and he pro- cured for me the jewellery designs shown on Plates IV and V.
If this book be found by those engaged in the jewellery trade helpful in their everyday work, and if it wakens in readers generally an appreciation of the variety of beautiful minerals suitable for gems, and an interest in the wondrous qualities of crystal- lized substances, I shall be more than satisfied.
G. F. H. S. WANDSWORTH COMMON, S.W.
CONTENTS
CHAP. PACK
I. INTRODUCTION ..... i
PART I— SECTION A THE CHARACTERS OF GEM-STONES
II. CRYSTALLINE FORM . . . . .6
III. REFLECTION, REFRACTION, AND DISPERSION . 14
IV. MEASUREMENT OF REFRACTIVE INDICES. . 21 V. LUSTRE AND SHEEN . . . .37
VI. DOUBLE REFRACTION . . . .40 VII. ABSORPTION EFFECTS : COLOUR, DICHROISM,
ETC 53
VIII. SPECIFIC GRAVITY . . . .63
IX. HARDNESS AND CLEAVABILITY . . .78
X. ELECTRICAL CHARACTERS . . . .82
PART I— SECTION B THE TECHNOLOGY OF GEM-STONES
XI. UNIT OF WEIGHT . . . . .84
XII. FASHIONING OF GEM-STONES . . .88
XIII. NOMENCLATURE OF PRECIOUS STONES . . 109
XIV. MANUFACTURED STONES . . . .113 XV. IMITATION STONES . . . . .124
ix
GEM-STONES
PART II— SECTION A
PRECIOUS STONES
CHAP.
XVI. DIAMOND.
XVII. OCCURRENCE OF DIAMOND . . • 137
XVIII. HISTORICAL DIAMONDS. . , • 157
XIX. CORUNDUM (Sapphire, Ruby) . . .172
XX. BERYL (Emerald, Aquamarine, Morganite) . 184
PART II— SECTION B SEMI-PRECIOUS STONES
XXI. TOPAZ 197
XXII. SPINEL (Balas-Ruby, Rubicelle) . . 203
XXIII. GARNET 207
(a) HESSONITE (Grossular, Cinnamon-Stone,
Hyacinth, Jacinth} . . .211
(b) PYROPE (' Cape- Ruby'} . . .212 (<r) RHODOLITE . . . .214
(d) ALMANDINE (Carbuncle) . . .214
(e) SPESSARTITE . . . .216 (/) ANDRADITE (Demantoid, Topazolite,
'Olivine'). . . . .216
(g) UVAROVITE . . . .218
XXIV. TOURMALINE (Rubellite) . . .219
XXV. PERIDOT . . . . . .225
XXVI. ZIRCON (Jargoon, Hyacinth, Jacinth] . . 228
XXVII. CHRYSOBERYL (Chrysolite, Cats-Eye, Cymo-
phane, Alexandrite) .... 233
XXVIII. QUARTZ (Rock- Crystal, Amethyst, Citrine,
Cairngorm, Cafs-Eye, Tigers-Eye) . . 238
XXIX. CHALCEDONY, AGATE, ETC. . . . 246
CONTENTS xi
XXX. OPAL (White Opal, Black Opal, Fire-Opal) 249 XXXI. FELSPAR (Moonstone, Sunstone, Labra-
dorite, Amazon-Stone) . . -254
XXXII. TURQUOISE, ODONTOLITE, VARISCITE . 257
XXXIII. JADE (Nephrite or Greenstone, Jadeite) . 260
XXXIV. SPODUMENE (Kunzite, Hiddenite], IOLITE,
BENITOITE . . . . .265
XXXV. EUCLASE, PHENAKITE, BERYLLONITE . 269 XXXVI. ENSTATITE ('Green Garnet'}, DIOPSIDE, KYANITE, ANDALUSITE, IDOCRASE, EPI- DOTE, SPHENE, AXINITE, PREHNITE, APATITE, DIOPTASE . . .271
XXXVII. CASSITERITE, ANATASE, PYRITES, HEMATITE 281 XXXVIII. OBSIDIAN, MOLDAVITE . . .283
PART II— SECTION C ORNAMENTAL STONES
XXXIX. FLUOR, LAPIS LAZULI, SODALITE, VIOLANE, RHODONITE, AZURITE, MALACHITE, THULITE, MARBLE, APOPHYLLITE, CHRYSOCOLLA, STEATITE OR SOAPSTONE, MEERSCHAUM, SERPENTINE . . 285
PART II— SECTION D ORGANIC PRODUCTS XL. PEARL, CORAL, AMBER . . .291
TABLES
I. CHEMICAL COMPOSITION OF GEM-STONES . 300 II. COLOUR OF GEM-STONES . . .301
III. REFRACTIVE INDICES OF GEM-STONES . 302
GEM-STONES
PAG!
IV. COLOUR-DISPERSION OF GEM-STONES . . 303 V. CHARACTER OF THE REFRACTION OF GEM- STONES . . . . . -303 VI. DICHROISM OF GEM-STONES . . . 304 VII. SPECIFIC GRAVITIES OF GEM-STONES . . 305 VIII. DEGREES OF HARDNESS OF GEM-STONES . 305 IX. DATA . . . . . . .306
INDEX ...... 307
LIST OF PLATES
PAGE
I. GEM-STONES (in colour) . . . Frontispiece
II. REFRACTIVE INDEX DIAGRAM. . . 36
III. INTERFERENCE FIGURES . . .48
IV. JEWELLERY DESIGNS . . . . 62 V. JEWELLERY DESIGNS . . . .88
VI. APPLIANCES USED FOR POLISHING DIAMONDS 102
VII. POLISHING DIAMONDS . . . .103
VIII. SLITTING AND POLISHING COLOURED STONES 104
IX. FACETING MACHINE . . . .105
X. LAPIDARY'S WORKSHOP AND OFFICE IN
ENGLAND . . . . 106
XL LAPIDARY'S WORKSHOP IN RUSSIA . . 107
XII. FRENCH FAMILY CUTTING STONES . . 108
XIII. INDIAN LAPIDARY . . . .109
XIV. BLOWPIPE USED FOR THE MANUFACTURE
OF RUBIES AND SAPPHIRES . . .118
XV. KlMBERLEY MlNE, 1 87 1 . . . .140
XVI. KlMBERLEY MINE, 1872. . . .141
XVII. KlMBERLEY MlNE, 1874. . . .142
XVIII. KlMBERLEY MlNE, l88l . . .143
XIX. KlMBERLEY MlNE AT THE PRESENT DAY . 144
XX. WESSELTON (open) MINE . . .145
XXI. LOADING THE BLUE GROUND ON THE
FLOORS, AND PLOUGHING IT OVER . . 146 XXII. WASHING-MACHINES FOR CONCENTRATING
THE BLUE GROUND . . . .147
XXIII. DIAMOND-SORTING MACHINES. . . 148 xiii
xiv GEM-STONES
PAGE
XXIV. KAFFIRS PICKING OUT DIAMONDS . . 149
XXV. CULLINAN DIAMOND (natural size) . . 168
XXVI. LARGE AQUAMARINE CRYSTAL (one-sixth natural size), FOUND AT MARAMBAYA, MINAS GERAES, BRAZIL . . . .196
XXVII. GEM-STONES (in colour) . . .226
XXVIII. OPAL MINES, WHITE CLIFFS, NEW SOUTH
WALES ...... 252
XXIX. GEM-STONES (in colour) . . .256
XXX. NATIVES DRILLING PEARLS . . . 294
XXXI. METAL FIGURES OF BUDDHA INSERTED IN A
PEARL-OYSTER . . . . 296
XXXII. SECTIONS OF CULTURE PEARL . . 297
GEM-STONES
GEM-STONES
CHAPTER 1 INTRODUCTION
BEAUTY, durability, and rarity: such are the three cardinal virtues of a perfect gem-stone. Stones lacking any of them cannot aspire to a high place in the ranks of precious stones, although it does not necessarily follow that they are of no use for ornamental purposes. The case of pearl, which, though not properly included among gem-stones, being directly produced by living agency, yet holds an honoured place in jewellery, constitutes to some extent an exception, since its incontestable beauty atones for its comparative want of durability.
That a gem-stone should be a delight to the eye is a truism that need not be laboured ; for such is its whole raison d'etre. The members of the Mineral Kingdom that find service in jewellery may be divided into three groups, according as they are transparent, translucent, or opaque. Of these the first, which is by far the largest and the most important, may itself be further sub-divided into two sections: stones which are devoid of colour, and stones which are tinted. Among the former, diamond reigns supreme, since it alone possesses
2 GEM-STONES
that marvellous ' fire,' oscillating with every move- ment from heavenly blue to glowing red, which is so highly esteemed and so much besought. Other stones, such as ' fired ' zircon, white sapphire, white topaz, and rock-crystal, may dazzle with brilliancy of light reflected from the surface or emitted from the interior, but none of them, like diamond, glow with mysterious gleams. No hint of colour, save perhaps a trace of the blue of steel, can be tolerated in stones of this category ; above all is a touch of the jaundice hue of yellow abhorred. It taxes all the skill of the lapidary to assure that the disposition of the facets be such as to reveal the full splendour of the stone. A coloured stone, on the other hand, depends for its attractiveness more upon its intrinsic hue than upon the manner of its cutting. The tint must not be too light or too dark in shade : a stone that has barely any colour has little interest, and one which is too dark appears almost opaque and black. The lapidary can to some extent remedy these defects by cutting the former deep and the latter shallow. In certain curious stones — for instance tourmaline — the transparency, and in others — such as ruby, sapphire, and one of the recent additions to the gem world, kunzite — the colour, varies considerably in different directions. The colours that are most admired — the fiery red of ruby, the royal blue of sapphire, the verdant green of emerald, and the golden yellow of topaz — are pure tints, and the absorption spectra corresponding to them are on the whole continuous and often restricted. They therefore retain the purity of their colour even in artificial light, though certain sapphires transmit a relatively larger amount of red,
INTRODUCTION 3
and consequently turn purple at night. Of the small group of translucent stones which pass light, but are not clear enough to be seen through, the most important is opal. It and certain others of the group owe their merit to the same optical effect as that characterizing soap-bubbles, tarnished steel, and so forth, and not to any intrinsic coloration. Another set of stones — moonstone and the star- stones — reflect light from the interior more or less regularly, but not in such a way as to produce a play of colour. The last group, which comprises opaque stones, has a single representative among ordinary gem-stones, namely, turquoise. In this case light is scattered and reflected from layers immediately contiguous to the surface, and the colour is due to the resulting absorption. The apparent darkness of a deep-coloured stone follows from a different cause : the light passing into the stone is wholly absorbed within it, and, since none is emitted, the stone appears black. The claims of turquoise are maintained by the blue variety ; there is little demand for stones of a greenish tinge.
It is evidently desirable that any stones used in jewellery should be able to resist the mechanical and chemical actions of everyday life. No one is anxious to replace jewels every few years, and the most valuable stones are expected to endure for all time. The mechanical abrasion is caused by the minute grains of sand that are contained in ordinary dust, and gem-stones should be at least as hard as they — a condition fulfilled by all the principal species with the exception of opal, turquoise, peridot, and demantoid. Since the beauty of the first named does not depend on the brilliancy of its
4 GEM-STONES
polish, scratches on the surface are not of much importance ; further, all four are only slightly softer than sand. It may be noted that the softness of paste stones, apart from any objections that may be felt to the use of imitations, renders them unsuitable for jewellery purposes. The only stones that are likely to be chemically affected in the course of wear are those which are in the slightest degree porous. It is hazardous to immerse turquoises in liquids, even in water, lest the bluish green colour be oxidized to the despised yellowish hue. The risk of damage to opals, moonstones, and star-stones by the penetration of dirt or grease into the interior of the stones is less, but is not wholly negligible. Similar remarks apply with even greater force to pearls. Their charm, which is due to a peculiar surface-play of light, might be destroyed by contamination with grease, ink, or similar matter ; they are, moreover, soft. For both reasons their use in rings is much to be deprecated. Nothing can be more unsightly than the dingy appearance of a pearl ring after a few years' wear.
It cannot be gainsaid that mankind prefers the rare to the beautiful, and what is within reach of all is lightly esteemed. It is for this reason that garnet and moonstone lie under a cloud. Purchasers can readily be found for a ' Cape-ruby ' or an 'olivine,' but not for a garnet; garnets are so common, is the usual remark. Nevertheless, the stones mentioned are really garnets. If science succeeded in manufacturing diamonds at the cost of shillings instead of the pounds that are now asked for Nature's products — not that such a prospect is at all probable or even feasible — we might expect them to vanish entirely from fashionable jewellery.
INTRODUCTION 5
A careful study of the showcases of the most extensive jewellery establishment brings to light the fact that, despite the apparent profusion, the number of different species represented is restricted. Diamond, ruby, emerald, sapphire, pearl, opal, turquoise, topaz, amethyst are all that are ordinarily asked for. Yet, as later pages will show, there are many others worthy of consideration ; two among them — peridot and tourmaline — are, indeed, slowly becoming known. For the first five of the stones mentioned above, the demand is relatively steady, and varies absolutely only with the purchasing power of the world ; but a lesser known stone may suddenly spring into prominence owing to the caprice of fashion or the preference of some great lady or leader of fashion. Not many years ago, for instance, violet was the favourite colour for ladies' dresses, and consequently amethysts were much worn to match, but with the change of fashion they speedily sank to their former obscurity. Another stone may perhaps figure at some royal wedding; for a brief while it becomes the vogue, and afterwards is seldom seen.
Except that diamond, ruby, emerald, and sapphire, and, we should add, pearl, may indis- putably be considered to occupy the first rank, it is impossible to form the gem-stones in any strict order. Every generation sees some change. The value of a stone is after all merely what it will fetch in the open market, and its artistic merits may be a matter of opinion. The familiar aphorism, de gustibus non est disputandum, is a warning not to enlarge upon this point.
PART I— SECTION A
THE CHARACTERS OF GEM- STONES
CHAPTER II CRYSTALLINE FORM
WITH the single exception of opal, the whole of the principal mineral species used in jewellery are distinguished from glass and similar substances by one fundamental difference : they are crystallized matter, and the atoms composing them are regularly arranged throughout the structure.
The words crystal and glass are employed in science in senses differing considerably from those in popular use. The former of them is derived from the Greek word «pvo?, meaning ice, and was at one time used in that sense. For instance, the old fourteenth-century reading of Psalm cxlvii. 1 7, which appears in the authorized version as " He giveth his ice like morsels," ran " He sendis his kristall as morcels." It was also applied to the beautiful, lustrous quartz found among the eternal snows of the Alps, since, on account of their limpidity, these stones were supposed, as Pliny tells us, to consist of water congealed by the extreme
CRYSTALLINE FORM 7
cold of those regions ; such at the present day is the ordinary meaning of the word. But, when early investigators discovered that a salt solution on evaporation left behind groups of slender glistening prisms, each very similar to the rest, they naturally — though, as we now know, wrongly — regarded them as representing yet another form of congealed water, and applied the same word to such substances. Subsequent research has shown that these salts, as well as mineral substances occurring with natural faces in nature, have in common the fundamental property of regularity of arrangement of the constituent atoms, and science therefore defines by the word crystal a substance in which the structure is uniform throughout, and all the similar atoms composing it are arranged with regard to the structure in a similar way.
The other word is yet more familiar ; it denotes the transparent, lustrous, hard, and brittle substance produced by the fusion of sand with soda or potash or both which fills our windows and serves a variety of useful purposes. Research has shown that glass, though apparently so uniform in character, has in reality no regularity of molecular arrange- ment. It is, in fact, a kind of mosaic of atoms, huddled together anyhow, but so irregular is its irregularity that it simulates perfect regularity. Science uses the word glass in this widened mean- ing. Two substances may, as a matter of fact, have the same chemical composition, and one be a crystal and the other a glass. For example, quartz, if heated to a high temperature, may be fused and converted into a glass. The difference in the two types of structure may be illustrated
8 GEM-STONES
by a comparison between a regiment of soldiers drawn up on parade and an ordinary crowd of people.
The crystalline form is a visible sign of the molecular arrangement, and is intimately associated with the directional physical properties, such as the optical characters, cleavage, etc. A study of it is not only of interest in itself, but also of great importance to the lapidary who wishes to cut a stone to the best advantage, and it is, moreover, of service in distinguishing stones when in the rough state.
The development of natural faces on a crystal
FIG. I.— Cubo-Octahedra.
is far from being haphazard, but is governed by the condition that the angles between similar faces, whether on the same crystal or on different crystals, are equal, however varying may be the shapes and the relative sizes of the faces (Fig. i), and by the tendency of the faces bounding the crystal to fall into series with parallel edges, such series being termed zones. Each species has a characteristic type of crystallization, which may be referred to one of the following six systems : —
I. Cubic. — Crystals in this system can be re- ferred to three edges, which are mutually at right angles, and in which the directional characters are identical in value. These principal edges are known
CRYSTALLINE FORM
as axes. Some typical forms are the cube (Fig. 2), characteristic of fluor ; the octahedron (Fig. 3), characteristic of diamond and spinel ; the dodeca- hedron (Fig. 4), characteristic of garnet; and the
FIG.
Cube. FIG. 3.— Octahedron. FIG. 4.— Dodecahedron.
triakisoctahedron, or three-faced octahedron (Fig. 5).
All crystals belonging to this system are singly refractive.
2. Tetragonal. — Such crystals can be referred
FIG. 5. — Triakis- octahedron, or Three-faced Oc- tahedron.
FIG. b. — Tetra- gonal Crystal.
to three axes, which are mutually at right angles, but in only two of them are the directional characters identical. A typical form is a four-sided prism, mm, of square section, terminated by four triangular faces,/* (Fig. 6), the usual shape of crystals of zircon and idocrase.
10
GEM-STONES
Crystals belonging to this system are doubly refractive and uniaxial, i.e. they have one direction of single refraction (cf. p. 45), which is parallel to the unequal edge of the three mentioned above.
H 3. Hexagonal.- — Such crystals
can be referred alternatively either to a set of three axes, X, Y, Z (Fig. 7), which lie in a plane perpendicular to a fourth, H, and are mutually inclined at angles of 60°, or to a set of three, a, b, c, which are not at FIG. 7.-T wo alternative right angles as in the cubic system, but in which the direc- tional characters are identical.
sets of Axes in the Hexagonal System.
The fourth axis in the first arrangement is equally inclined to each in the second set of axes. Many important species crystallize in this system — corundum (sapphire, ruby), beryl (emerald, aqua- marine), tourmaline, quartz, and phenakite. The crystals usually
FIGS. 8-10. — Hexagonal Crystals.
display a six-sided prism, terminated by a single face, c, perpendicular to the edge of the prism m (Fig. 8), e.g. emerald, or by six or twelve inclined faces, p (Fig. 9), e.g. quartz, crystals of which are
CRYSTALLINE FORM
1 1
so constant in form as to be the most familiar in the Mineral Kingdom. Tourmaline crystals (Fig. 10) are peculiar because of the fact that often one end is obviously to the eye flatter than the other.
Crystals belonging to this system are also doubly refractive and uniaxial, the direction of single refraction being parallel to the fourth axis mentioned above, and therefore also parallel to the prism edge. Hence deeply coloured tourmaline, which strongly absorbs the ordinary ray, must be cut with the table-facet parallel to the edge of the prism.
4. Orthorhombic. — Such crystals can be referred to three axes, which are mutu- ally at right angles, but in which each of the directional characters are different. The
crystals are usually prismatic FlG> Vi. -Relation of the
in shape, One of the axes two directions of single
being parallel to the prism Refraction to the Axes
, _ • 1 . j in an Orthorhombic
edge. Topaz, peridot, and Crystal
chrysoberyl are the most
important species crystallizing in this system.
Crystals belonging to this system are doubly refractive and biaxial, i.e. they have two directions of single refraction (cf. p. 45). The three axes a, d, c (Fig. 1 1) are parallel respectively to the two bisectrices of the directions of single refraction, and the direction perpendicular to the plane con- taining those directions.
5. Monoclinic. — Such crystals can be referred to three axes, one of which is at right angles to the other two, which are, however, themselves not at
12 GEM-STONES
right angles. Spodumene (kunzite) and some moonstone crystallize in this system.
Crystals belonging to this system are doubly refractive and biaxial, but in this case the first axis alone is parallel to one of the principal optical directions.
6. Tridinic. — Such crystals have no edges at right angles, and the optical characters are not immediately related to the crystalline form. Some moonstone crystallizes in this system.
Crystals are often not single separate individuals. For instance, diamond and spinel are found in fiat triangular crystals with their girdles cleft at the corners (Fig. 1 2). Each of such crystals is really composed of portions of two similar octahedra, which are placed together in such a way that each is a reflection of FIG. i2.— Twinned the other. Such composite crystals Octahedron. are called twins or macles. Some- times the twinning is repeated, and the individuals may be so minute as to call for a microscope for their perception.
A composite structure may also result from the conjunction of numberless minute individuals without any definite orientation, as in the case of chalcedony and agate. So by supposing the individuals to become infinitesimally small, we pass to a glass-like substance.
It is often a peculiarity of crystals of a species to display a typical combination of natural faces. Such a combination is known as the habit of the species, and is often of service for the purpose of identifying stones before they are cut. Thus, a
CRYSTALLINE FORM 13
habit of diamond and spinel is an octahedron, often twinned, of garnet a dodecahedron, of emerald a flat-ended hexagonal prism, and so on.
It is one of the most interesting and remarkable features connected with crystallization that the composition and the physical characters — for instance, the refractive indices and specific gravity — may, without any serious disturbance of the molecular arrangement, vary considerably owing to the more or less complete replacement of one element by another closely allied to it. That is the cause of the range of the physical characters which has been observed in such species as tourmaline, peridot, spinel, etc. The principal replacements in the case of the gem-stones are ferric oxide, Fe2O3, by alumina, A12O3, and ferrous oxide, FeO, by magnesia, MgO.
A list of the principal gem-stones, arranged by their chemical composition, is given in Table I at the end of the book.
CHAPTER III REFLECTION, REFRACTION, AND DISPERSION
IT is obvious that, since a stone suitable for ornamental use must appeal to the eye, its most important characters are those which depend upon light ; indeed, the whole art of the lapidary consists in shaping it in such a way as to show these qualities to the best advantage. To under- stand why certain forms are given to a cut stone, it is essential for us to ascertain what becomes of the light which falls upon the surface of the stone ; further, we shall find that the action of a stone upon light is of very great help in distinguishing the different species of gem-stones. The phenomena displayed by light which impinges upon the surface separating two media1 are very similar in character, whatever be the nature of the media.
Ordinary experience with a plane mirror tells us that, when light is returned, or reflected, as it is usually termed, from a plane or flat surface, there is no alteration in the size of objects viewed in this way, but that the right and the left hands are inter- changed : our right hand becomes the left hand in
1 The word medium is employed by physicists to express any sub- stance through which light passes, and includes solids such as glass, liquids such as water, and gases such as air ; the nature of the substance is not postulated.
REFLECTION, REFRACTION, DISPERSION 15
our reflection in the mirror. We notice, further, that our reflection is apparently just as far distant from the mirror on the farther side as we are on this side. In Fig. 1 3 MM' is a section of the mirror, and O' is the image of the hand O as seen in the mirror. Light from O reaches the eye E by way of m, but it appears to come from O. Since OO is perpendicular to the mirror, and O and O lie at equal distances from it, it follows from elementary
FIG. 13.— Reflection at a Plane Mirror.
geometry that the angle z", which the reflected ray makes with win, the normal to the mirror, is equal to 2, the angle which the incident ray makes with the same direction.
Again, everyday experience tells us that the case is less simple when light actually crosses the bounding surface and passes into the other medium. Thus, if we look down into a bath filled with water, the bottom of the bath appears to have been raised up, and a stick plunged into the water seems to be
1 6 GEM-STONES
bent just at the surface, except in the particular case when it is perfectly upright. Since the stick itself has not been bent, light evidently suffers some change in direction as it passes into the water or emerges therefrom. The passage of light from one medium to another was studied by Snell in the seventeenth century, and he enunciated the follow- ing laws : —
1. The refracted ray lies in the plane containing the incident ray and the normal to the plane surface separating the two media.
It will be noticed that the reflected ray obeys this law also.
2. The angle r, which the refracted ray makes with the normal, is related to the angle z, which the incident ray makes with the same direction, by the equation
n sin i = n sin r, (a)
where n and n are constants for the two media which are known as the indices of refraction, or the refractive indices.
This simple trigonometrical relation may be ex- pressed in geometrical language. Suppose we cut a plane section through the two media at right angles to the bounding plane, which then appears as a straight line, SOS' (Fig. 14), and suppose that IO represents the direction of the incident ray ; then Snell's first law tells us that the refracted ray OR will also lie in this plane. Draw the normal NON1, and with centre O and any radius describe a circle intersecting the incident and refracted rays in the points a and b respectively ; let drop perpendiculars ac and bd on to the normal NON'. Then we have
REFLECTION, REFRACTION, DISPERSION if
n .ac—ri . bd, whence we see that if n be greater than «', ac is less than bd, and therefore when light passes from one medium into another which is less optically dense, in its passage across the boundary it is bent, or refracted, away from the normal.
We see, then, that when light falls on the boundary of two different media, some is reflected in the first and some is refracted into the second medium.
N
FIG. 14.— Refraction across a Plane Surface.
The relative amounts of light reflected and refracted depend on the angle of incidence and the refractive indices of the media. We shall return to this point when we come to consider the lustre of stones.
We will proceed to consider the course of rays at different angles of incidence when light passes out from a medium into one less dense — for instance, from water into air. Corresponding to light with a small angle of incidence such as I^O (Fig. 15), some of it is reflected in the direction OI\ and the
2
i8
GEM-STONES
remainder is refracted out in the direction ORV Similarly, for the ray 72<9 some is reflected along 0/2 and some refracted along ORZ. Since, in the case we have taken, the angle of refraction is greater than the angle of incidence, the refracted ray corresponding to some incident, ray ICO will graze the bounding surface, and corresponding to
Ic
FIG. 15. — Total -Reflection.
a ray beyond it, such as 73(9, which has a still greater angle of incidence, there is no refracted ray, and all the light is wholly or totally reflected within the dense medium. The critical angle ICON, which is called the angle of total-reflection, is very simply related to the refractive indices of the two media ; for, since r is now a right angle, sin r= i, and equa- tion (a) becomes
n sin i — n' . (b\
REFLECTION, REFRACTION, DISPERSION 19
Hence, if the angle of total-reflection is measured and one of the indices is known, the other can easily be calculated.
The phenomenon of total-reflection may be ap- preciated if we hold a glass of water above our head, and view the light of a lamp on a table reflected from the under surface of the water. This reflection is incomparably more brilliant than that given by the upper surface.
The refractive index of air is taken as unity ; strictly, it is that of a vacuum, but the difference is too small to be appreciated even in very delicate work. Every substance has different indices for light of different colour, and it is customary to take as the standard the yellow light of a sodium flame. This happens to be the colour to which our eyes are most sensitive, and a flame of this kind is easily prepared by volatilizing a little bicarbonate of soda in the flame of a bunsen burner. A survey of Table III at the end of the book shows clearly how valuable a measurement of the refractive index is for determining the species to which a cut stone belongs. The values found for different specimens of the species do in cases vary considerably owing to the great latitude possible in the chemical con- stitution due to the isomorphous replacement of one element by another. Some variation in the index may even occur in different directions within the same stone ; it results from the remarkable property of splitting up a beam of light into two beams, which is possessed by many crystallized substances. This forms the subject of a later chapter.
Upon the fact that the refractive index of a
20 GEM-STONES
substance varies for light of different colours depends such familiar phenomena as the splendour of the rainbow and the ' fire ' of the diamond. When white light is refracted into a stone it no longer remains white, but is split up into a spectrum. Except in certain anomalous substances the refractive index increases progressively as the wave-length of the light decreases, and consequently a normal spectrum is violet at one end and passes through green and yellow to red at the other end. The width of the spectrum, which may be measured by the difference between the refractive indices for the extreme red and violet rays, also varies, though on the whole it increases with the refractive index. It is the dispersion, as this difference is termed, that determines the ' fire ' — a character of the utmost importance in colourless transparent stones, which, but for it, would be lacking in interest. Diamond excels all colourless stones in this respect, although it is closely followed by zircon, the colour of which has been driven off by heating ; it is, however, sur- passed by two coloured species : sphene, which is seldom seen in jewellery, and demantoid, the green garnet from the Urals, which often passes under the misnomer ' olivine.' The dispersion of the more prominent species for the B and G lines of the solar spectrum is given in Table IV at the end of the book.
We will now proceed to discuss methods that may be used for the measurement of the refractive indices of cut stones.
CHAPTER IV MEASUREMENT OF REFRACTIVE INDICES
THE methods available for the measurement of refractive indices are of two kinds, and make use of two different principles. The first, which is based upon the very simple relation found in the last chapter to subsist at total-reflection, can be used with ease and celerity, and is best suited for discriminative purposes ; but it is re- stricted in its application. The second, which depends on the measurement of the angle between two facets and the minimum deviation experienced by a ray of light when traversing a prism formed by them, is more involved, entails the use of more elaborate apparatus, and takes considerable time, but it is less restricted in its application.
(i) THE METHOD OF TOTAL-REFLECTION
We see from equation b (p. 1 8), connecting the angle of total-reflection with the refractive indices of the adjacent media, that, if the denser medium be constant, the indices of all less dense media may be easily determined from a measurement of the corresponding critical angle. In all refracto- meters the constant medium is a glass with a high refractive index. Some instruments have rotatory
22 GEM-STONES
parts, by means of which this angle is actually measured. Such instruments give very good results, but suffer from the disadvantages of being neither portable nor convenient to handle, and of not giving a result without some computation.
For use in the identification of cut stones, a refractometer with a fixed scale, such as that (Fig. 1 6) devised by the author, is far more convenient. In order to facilitate the observations, a totally reflecting prism has been inserted between the two
FlG. 16. — Refractometer (actual size).
lenses of the eyepiece. The eyepiece may be adjusted to suit the individual eyesight; but for observers with exceptionally long sight an adapter is provided, which permits the eyepiece being drawn out to the requisite extent. The refracto- meter must be held in the manner illustrated in Fig. 17, so that the light from a window or other source of illumination enters the instrument by the lenticular opening underneath. Good, even illumina- tion of the field may also very simply be secured by reflecting light into the instrument from a sheet
REFRACTIVE INDICES 23
of white paper laid on a table. On looking down the eyepiece we see a scale (Fig. 1 8), the eyepiece being, if necessary, focused until the divisions of the scale are clearly and distinctly seen. Suppose, for experiment, we smear a little vaseline or similar fatty substance on the plane surface of the dense glass, which just projects beyond the level of the
FIG. 17. — Method of Using the Refractometer.
brass plate embracing it. The field of view is now no longer uniformly illuminated, but is divided into two parts (Fig. 19): a dark portion above, which terminates in a curved edge, apparently green in colour, and a bright portion underneath, which is composed of totally reflected light. If we tilt the instrument downwards so that light enters the instrument from above through the vaseline we find that the portions of the field are
GEM-STONES
reversed, the dark portion being underneath and terminated by a red edge. It is possible so to arrange the illumination that the two portions are evenly lighted, and the common edge becomes almost invisible. It is therefore essential for obtaining satisfactory results that the plate and the dense glass be shielded from the light by the
IEFFWCTIVE INDEX
-= 1-30
m 1-35
fH 1-40
fJH 1-45
= 1-50
HI 1-55
= 1-60
1-70
= 1-75
FIG. 1 8. —Scale of the Refrac- tometer.
FIG. 19.— Shadow, edge given by a singly refractive Substance.
disengaged hand. The shadow-edge is curved, and is, indeed, an arc of a circle, because spherical surfaces are used in the optical arrangements of the refractometer ; by the substitution of cylindrical surfaces it becomes straight, but sufficient advantage is not secured thereby to compensate for the greatly increased complexity of the construction. The shadow-edge is coloured, because the relative
dispersion, — (nv and nr being the refractive
REFRACTIVE INDICES 25
indices for the extreme violet and red rays respectively), of the vaseline differs from that of the dense glass. The dispersion of the glass is very high, and exceeds that of any stone for which it can be used. Certain oils have, however, nearly the same relative dispersion, and the edges corresponding to them are consequently almost colourless. A careful eye will perceive that the coloured shadow-edge is in reality a spectrum, of which the violet end lies in the dark portion of the field and the red edge merges into the bright portion. The yellow colour of a sodium flame, which, as has already been stated, is selected as the standard for the measurement of refractive indices, lies between the green and the red, and the part of the spectrum to be noted is at the bottom of the green, and practically, therefore, at the bottom of the shadow, because the yellow and red are almost lost in the brightness of the lower portion of the field. If a sodium flame be used as the source of illumination, the shadow-edge becomes a sharply defined line. The scale is so graduated and arranged that the reading where this line crosses the scale gives the corresponding refractive index, the reading, since the line is curved, being taken in the middle of the field on the right-hand side of the scale. The refracto- meter therefore gives at once, without any inter- mediate calculation, a value of the refractive index to the second place of decimals, and a skilled observer may, by estimating the tenths of the intervals between successive divisions, arrive at the third place ; to facilitate this estimation the semi-divisions beyond 1-650 have been inserted.
26 GEM-STONES
The range extends nearly to r8oo; for any substance with a higher refractive index the field is dark as far as the limit at the bottom.
A fat, or a liquid, wets the glass, i.e, comes into intimate contact with it, but if a solid substance be tested in the same way, a film of air would intervene and entirely prevent an observation. To displace it, a drop of some liquid which is more highly refractive than the substance under test must first be applied to the plane surface of the dense glass. The most convenient liquid for the purpose is methylene iodide, CH2I2, which, when pure, has at ordinary room temperatures a refrac- tive index of 1*742. It is almost colourless when fresh, but turns reddish brown on exposure to light. If desired, it may be cleared in the manner described below (p. 66), but the film of liquid actually used is so thin that this precaution is scarcely necessary. If we test a piece of ordinary glass — one of the slips used by microscopists for covering thin sections is very convenient for the purpose — first applying a drop of methylene iodide to the plane surface of the dense glass of the refractometer (Fig. 20), we notice a coloured shadow-edge corresponding to the glass- slip at about i'53O and another, almost colourless, at 1742, which corresponds to the liquid. If the solid substance which is tested is more highly refractive than methylene iodide, only the latter of the shadow-edges is visible, and we must utilize some more refractive liquid. We can, however, raise the refractive index of methylene iodide by dissolving sulphur l in it ; the refractive index of
1 Methylene iodide must be heated almost to boiling-point to enable it to absorb sufficient sulphur ; but caution must be exercised in the
REFRACTIVE INDICES 27
the saturated liquid lies well beyond i'8oo, and the shadow-edge corresponding to it, therefore, does not come within the range of the refractometer. The pure and the saturated liquids can be procured with the instrument, the bottles containing them being japanned on the outside to exclude light and fitted with dipping-stoppers, by means of which a drop of the liquid required is easily transferred to the surface of the glass of the instrument. So long
Stone
FIG. 20. — Faceted Stone in Position on the Refractometer.
as the liquid is more highly refractive than the stone, or whatever may be the substance under examination, its precise refractive index is of no consequence. The facet used in the test must be flat, and must be pressed firmly on the instrument, so that it is truly parallel to the plane surface of the dense glass ; for good results, moreover, it must be bright.
operation to prevent the liquid boiling over and catching fire, the resulting fumes being far from pleasant. It is advisable to verify by actual observation that the liquid is refractive enough not to show any shadow-edge in the field of view of the refractometer.
28 GEM-STONES
We have so far assumed that the substance which we are testing is simple and gives a single shadow-edge; but, as may be seen from Table V, many of the gem-stones are doubly refractive, and such will, in general, show in the field of the refractometer two distinct shadow- edges more or less widely separated. Suppose, for example, we study the effect produced by a peridot, which displays the phenomenon to a marked degree. If we revolve the stone so that the facet under observation remains parallel to the plane surface of the dense glass of the refracto- meter and in contact with it, we notice that both the shadow-edges in general move up or down the scale. In particular cases, depending upon the relation of the position of the facet selected to the crystalline symmetry, one or both of them may remain fixed, or one may even move across the other. But whatever facet of the stone be used for the test, and however variable be the movements of the shadow -edges, the highest and lowest readings obtainable remain the same; they are the principal indices of refraction, such as are stated in Table III at the end of the book, and their difference measures the maximum amount of double refraction possessed by the stone. The procedure is therefore simplicity itself; we have merely to revolve the stone on the instrument, usually through not more than a right angle, and note the greatest and least readings. It will be noticed that the shadow-edges cross the scale symmetrically in the critical and skewwise in intermediate positions. Fig. 21 represents the effect when the facet is such as to give simul-
REFRACTIVE INDICES
29
taneously the two readings required. The shadow- edges a and b, which are coloured in white light, correspond to the least and greatest respectively of the principal refractive indices, while the third shadow-edge, which is very faint, corresponds to the liquid used — methylene iodide. It is possible, as we shall see in a later chapter, to learn from the motion, if any, of the shadow- edges something as to the character of the double refraction. Since, however, each shadow-edge is spec- tral in white light, they will not be distinctly separate unless the double refraction exceeds the relative dis- persion. Topaz, for instance, ap- pears in white light to yield only a single shadow-edge, and may thus easily be distinguished from tour- maline, in which the double re- fraction is large enough for the separation of the two shadow-edges to be clearly discerned. In sodium light, however, no difficulty is ex- perienced in distinguishing both the shadow-edges given by substances with small amount of double refraction, such as chrysoberyl, quartz, and topaz, and a skilled observer may detect the separa- tion in the extreme instances of apatite, idocrase, and beryl. The shadow-edge corresponding to the greater refractive index is always less distinct, because it lies in the bright portion of the field. If the stone or its facet be small, it must be moved on the plane surface of the dense glass until the greatest possible distinctness is imparted to the
FIG. 21.— Shadow- edges given by a doubly refractive Substance.
30 GEM-STONES
edge or edges. If it be moved towards the observer from the further end, a misty shadow appears to move down the scale until the correct position is reached, when the edges spring into view.
Any facet of a stone may be utilized so long as it is flat, but the table-facet is the most convenient, because it is usually the largest, and it is available even when the stone is mounted. That the stone need not be removed from its setting is one of the great advantages of this method. The smaller the stone the more difficult it is to manipulate ; caution especially must be exercised that it be not tilted, not only because the shadow-edge would be shifted from its true position and an erroneous value of the refractive index obtained, but also because a corner or edge of the stone would inevitably scratch the glass of the instrument, which is far softer than the hard gem-stones. Methylene iodide will in time attack and stain the glass, and must therefore be wiped off the instru- ment immediately after use.
(2) THE METHOD OF MINIMUM DEVIATION
If the stone be too highly refractive for a measurement of its refractive index to be possible with the refractometer just described, and it is desired to determine this constant, recourse must be had to the prismatic method, for which purpose an instrument known as a goniometer l is required.
1 yuvla, angle ; /j.4rpov, measure. For details of the construction, adjustment, and use of this instrument the reader should refer to text- books of mineralogy or crystallography.
REFRACTIVE INDICES 31
Two angles must be measured ; one the interior angle included between a suitable pair of facets, and the other the minimum amount of the deviation produced by the pair upon a beam of light traversing them.
Fig. 22 represents a section of a step-cut stone perpendicular to a series of facets with parallel edges ; / is the table, and a, b, c, are facets on the culet side. The path of light traversing the prism formed by the pair of facets, / and b, is indicated. Suppose that A is the interior angle of the prism, i the angle of incidence of light at the first facet and if the angle of emergence at the second facet, and r and / the angles inside the stone at the two facets respectively. Then at the FIG. 22.— Path at Minimum De- first facet light has been bent through an angle a Cut Stone"." i — r, and again at the
second facet through an angle i' - / ; the angle of deviation, D, is therefore given by
We have further that
whence it follows that
A + D~i+?.
If the stone be mounted on the goniometer
32 GEM-STONES
and adjusted so that the edge of the prism is parallel to the axis of rotation of the instrument and if light from the collimator fall upon the table-facet and the telescope be turned to the proper position to receive the emergent beam, a spectral image of the object-slit, or in the case of a doubly refractive stone in general, two spectral images, will be seen in white light ; in the light of a sodium flame the images will be sharp and distinct. Suppose that we rotate the stone in the direction of diminishing deviation and simul- taneously the telescope so as to retain an image in the field of view, we find that the image moves up to and then away from a certain position, at which, therefore, the deviation is a minimum. The image moves in the same direction from this position whichever way the stone be rotated. The question then arises what are the angles of incidence and refraction under these special conditions. It is clear that a path of light is reversible ; that is to say, if a beam of light traverses the prism from the facet t to the facet b, it can take precisely the same path from the facet b to the facet t. Hence we should be led to expect that, since experiment teaches us that there is only one position of minimum deviation corre- sponding to the same pair of facets, the angles at the two facets must be equal, i.e. i=if, and r—S. It is, indeed, not difficult to prove by either geometrical
or analytical methods that such is the case.
^0
Therefore at minimum deviation r=— and
2
. A+D . .
t = , and, since sin t = n sm r, where « is
REFRACTIVE INDICES 33
the refractive index of the stone, we have the simple relation —
This relation is strictly true only when the direction of minimum deviation is one of crystal- line symmetry in the stone, and holds therefore in general for all singly refractive stones, and for the ordinary ray of a uniaxial stone ; but the values thus obtained even in the case of biaxial stones are approximate enough for discriminative purposes. If then the stone be singly refractive, the result is the index required ; if it be uniaxial, one value is the ordinary index and the other image gives a value lying between the ordinary and the extraordinary indices ; if it be biaxial, the values given by the two images may lie anywhere between the greatest and the least refractive indices. The angle A must not be too large ; otherwise the light will not emerge at the second facet, but will be totally reflected inside the stone : on the other hand, it must not be too small, because any error in its determination would then seriously affect the accuracy of the value derived for the refractive index. Although the monochromatic light of a sodium flame is essential for precise work, a sufficiently approximate value for discriminative purposes is obtained by noting the position of the yellow portion of the spectral image given in white light.
In the case of a stone such as that depicted in Fig. 2 2 images are given by other pairs of facets, for 3
34
GEM-STONES
instance ta and tc, unless the angle included by the former is too large. There might therefore be some doubt, to which pair some particular image corresponded; but no confusion can arise if the following procedure be adopted.
The table, or some easily recognizable facet, is selected as the facet at which light enters the stone. The telescope is first placed in the position in which it is directly opposite the collimator (T0 in Fig. 23), and clamped. The scale is turned
until it reads ex- actly zero, o° or 360°, and clamped. The telescope is re- leased and revolved in the direction of T* increasing readings of the scale to the position of minimum deviation, T. The
reading of the scale FIG. 23. — Course of Observations in the .
Method of Minimum Deviation. glves at once the
angle of minimum
deviation, D. The holder carrying the stone is now clamped to the scale, and the telescope is turned to the position, 7\, in which the image given by reflection from the table facet is in the centre of the field of view; the reading of the scale is taken. The telescope is clamped, and the scale is released and rotated until it reads the angle already found for D. If no mistake has been made, the reflected image from the second facet is now in the field of view. It will probably not be quite central, as theoretically it should be, because the
REFRACTIVE INDICES 35
stone may not have been originally quite in the position of minimum deviation, a comparatively large rotation of the stone producing no apparent change in the position of the refracted image at minimum deviation, and further, because, as has already been stated, the method is not strictly true for biaxial stones. The difference in readings, however, should not exceed 2°. The reading, S, of the scale is now taken, and it together with 180° subtracted from the reading for the first facet, and the value of A, the interior angle between the two facets, obtained.
Let us take an example.
Reading T ( = /?) 40° 41' Reading 7\ 261° 35'
less 1 80° i So o
8i 35 Reading 5" 41 30
\D 20 2oJ A 40
\A 20 2\ \A 20
o 23
Log sin 40° 23' 9.81151 Log sin 20 2| 9.53492
Log n 0.27659
n= 1.8906.
The readings 5" and T are very nearly the same, and therefore we may be sure that no mistake has been made in the selection of the facets.
In place of logarithm-tables we may make use of the diagram on Plate II. The radial lines
36 GEM-STONES
correspond to the angles of minimum deviation and the skew lines to the prism angles, and the distance along the radial lines gives the refractive index. We run our eye along the line for the observed angle of minimum deviation and note where it meets the curve for the observed prism angle ; the refractive index corresponding to the point of intersection is at once read off.
This method has several obvious disadvantages : it requires the use of an expensive and elaborate instrument, an observation takes considerable time, and the values of the principal refractive indices cannot in general be immediately determined.
Table III at the end of the book gives the refractive indices of the gem-stones.
Prism-angle
CHAPTER V LUSTRE AND SHEEN
IT has been already stated that whenever light in one medium falls upon the surface separating it from another medium some of the light is reflected within the first, while the remainder passes out into the second medium, except when the first is of lower refractivity than the second and light falls at an angle greater than that of total-reflection. Similarly, when light impinges upon a cut stone some of it is reflected and the remainder passes into the stone. What is the relative amount of reflected light depends upon the nature of the stone — its refractivity and hardness — and determines its lustre ; the greater the amount the more lustrous will the stone appear. There are different kinds of lustre, and the intensity of each depends on the polish of the surface. From a dull, i.e. an uneven, surface the reflected light is scattered, and there are no brilliant reflections. All gem-stones take a good polish, and have therefore, so long as the surface retains its polish, considerable brilliancy; turquoise, on account of its softness, is always comparatively dull. The different kinds of lustre are —
(1) Adamantine, characteristic of diamond.
(2) Vitreous, as seen on the surface of
fractured glass.
(3) Resinous, as shown by resins.
38 GEM-STONES
Zircon and demantoid, the green garnet called by jewellers " olivine," alone among gem-stones have a lustre approaching that of diamond. The remainder all have a vitreous lustre, though varying in degree, the harder and the more refractive species being on the whole the more lustrous.
Some stones — for instance, a cinnamon garnet — appear to have a certain greasiness in the lustre, which is caused by stray reflections from inclusions or other breaks in the homogeneity of the interior. A pearly lustre, which arises from cleavage cracks and is typically displayed by the cleavage face of topaz, would be seen in a cut stone only when flawed.
Certain corundums when viewed in the direction of the crystallographical axis display six narrow lines of light radiating at angles of 60° from a centre in a manner suggestive of the conventional representations of stars. Such stones are con- sequently known as asterias, or more usually star- stones — star-rubies or star-sapphires, as the case may be, and the phenomenon is called asterism. These stones have not a homogeneous structure, but contain tube-like cavities regularly arranged at angles of 60* in planes at right angles to the crystallographical axis. The effect is best produced when the stones are cut en cabochon perpendicular to that axis.
Chatoyancy is a somewhat similar phenomenon, but in this case the fibres or cavities are parallel to a single direction, and a single broadish band is displayed at right angles to it. Cat's-eyes, as these stones are termed, are cut en cabochon parallel to the fibres. The true cat's-eye (Plate XXIX, Fig. i)
LUSTRE AND SHEEN 39
is a variety of chrysoberyl, but the term is also often applied to quartz showing a similar appearance. The latter is really a fibrous mineral, such as asbestos, which has become converted into silica. The beautiiul tiger's-eye from South Africa is a silicified crocidolite, the original blue colour of which has been altered by oxidation to golden brown. Recently tourmalines have been discovered which are sufficiently fibrous in structure to display an effective chatoyancy.
The milky sheen of moonstone (Plate XXIX, Fig. 4) owes its effect to reflections from twin lamellae. The wonderful iridescence which is the glory of opal, and is therefore termed opalescence, arises from a structure which is peculiar to that species. Opal is a solidified jelly ; on cooling it has become riddled with extremely thin cracks, which were subsequently filled with similar material of slightly different refractivity, and thus it consists of a series of films. At the surface of each film interference of light takes place just as at the surface of a soap-bubble, and the more evenly the films are spaced apart the more uniform is the colour displayed, the actual tint depending upon the thickness of the films traversed by the light giving rise to the phenomenon.
CHAPTER VI DOUBLE REFRACTION
r I ^HE optical phenomenon presented by many J. gem-stones is complicated by their property of splitting up a beam of light into two with, in general, differing characters. In this chapter we shall discuss the nature of double refraction, as it is termed, and methods for its detection. The pheno- menon is not one that comes within the purview of everyday experience.
So long ago as 1669 a Danish physician, by name Bartholinus, noticed that a plate of the trans- parent mineral which at that time had recently been brought over from Iceland, and was therefore called " Iceland-spar," possessed the remarkable property of giving a double image of objects close to it when viewed through it. Subsequent investigation has shown that much crystallized matter is doubly refractive, but in calcite — to use the scientific name for the species which includes Iceland-spar — alone among common minerals is the phenomenon so conspicuous as to be obvious to the unaided eye. The apparent separation of the pair of images given by a plate cut or cleaved in any direction depends upon its thickness. The large mass, upwards of two feet (60 cm.) in thickness, which is exhibited at the far end of the Mineral Gallery of the British
DOUBLE REFRACTION 41
Museum (Natural History), displays the separation to a degree that is probably unique.
Although none of the gem-stones can emulate calcite in this character, yet the double refraction of certain of them is large enough to be detected without much difficulty. In the case of faceted stones the opposite edges should be viewed through the table-facet, and any signs of doubling noted.
FIG. 24. — Apparent doubling of the Edges of a Peridot when viewed through the Table-Facet.
The double refraction of sphene is so large, viz. O'O8, that the doubling of the edges is evident to the unaided eye. In peridot (Fig. 24), zircon (b), and epidote the apparent separation of the edges is easily discerned with the assistance of an ordinary lens. A keen eye can detect the phenomenon even in the case of such substances as quartz with small double refraction. It must, however, be remembered that in all such stones the refraction is single in certain directions, and the amount of double refraction
42 GEM-STONES
varies therefore with the direction from nil to the maximum possessed by the stone. Experiment with a plate of Iceland-spar shows that the rays transmitted by it have properties differing from those of ordinary light On superposing a second plate we notice that there are now two pairs of images, which are in general no longer of equal brightness, as was the case before. If the second plate be rotated with respect to the first, two images, one of each pair, disappear, and then the other two, the plate having turned through a right angle between the two positions of extinction ; midway between these positions the images are all equally
a FIG. 25. — Wave-Motion.
bright This variation of intensity implies that each of the rays emerging from the first plate has acquired a one-sided character, or, as it is usually expressed, has become plane-polarized, or, shortly, polarized.
Before the discovery of the phenomenon of double refraction the foundation of the modern theory of light had been laid by the genius of Huygens. According to this theory light is the result of a wave-motion (Fig. 25) in the ether, a medium that pervades the whole of space whether occupied by matter or not, and transmits the wave-motion at a rate varying with the matter with which it happens to coincide. Such a medium has been assumed
DOUBLE REFRACTION 43
because it explains satisfactorily all the phenomena of light, but it by no means follows that it has a concrete existence. Indeed, if it has, it is so tenuous as to be imperceptible to the most delicate experiments. The wave-motion is similar to that observed on the surface of still water when disturbed by a stone flung into it. The waves spread out from the source of disturbance; but, although the waves seem to advance, the actual particles of water merely move up and down, and have no motion at all in the direction in which the waves are moving. If we imagine similar motion to take place in any plane and not only the horizontal, we form some idea of the nature of ordinary light. But after passing through a plate of Iceland-spar, light no longer vibrates in all directions, but in each beam the vibrations are parallel to a particular plane, the two planes being at right angles. The exact relation of the direction of the vibrations to the plane of polariz- ation is uncertain, although it undoubtedly lies in the plane containing the direction of the ray of light and the perpendicular to the plane of polarization. The waves for different colours differ in their length, i.e. in the distance, 2 bb (Fig. 25), from crest to crest, while the velocity, which remains the same for the same medium, is proportional to the wave-length. The intensity of the light varies as the square of the amplitude of the wave, i.e. the height, ab, of the crest from the mean level.
Various methods have been proposed for obtain- ing polarized light. Thus Seebeck found in 1813 that a plate of brown tourmaline cut parallel to the crystallographic axis and of sufficient thickness (cf. p. n) transmits only one ray, the other being
44 GEM-STONES
entirely absorbed within the plate. Another method was to employ a glass plate to reflect light at a certain critical angle. The most efficient method, and that in general use at the present day, is due to the invention of Nicol. A rhomb of Iceland- spar (Fig. 26), of suitable length, is sliced along the longer diagonal, dd, and the halves are cemented together by means of Canada balsam. One ray, ioo, is totally reflected at the surface separating the mineral and the cement, and does not penetrate into the other half; while the other ray, iee, is trans- mitted with almost undiminished intensity. Such
FIG. 26.— Nicol's Prism.
a rhomb is called a Nicol's prism after its inventor, or briefly, a nicol.
If one nicol be placed above another and their corresponding principal planes be at right angles no light is transmitted through the pair. In the polarizing microscope one such nicol, called the polarizer, is placed below the stage, and the other, called the analyser, is either inserted in the body of the microscope or placed above the eyepiece, and the pair are usually set in the crossed position so that the field of the microscope is dark. If a piece of glass or a fragment of some singly refractive sub- stance be placed on the stage the field still remains
DOUBLE REFRACTION 45
dark ; but in case of a doubly refractive stone the field is no longer dark except in certain positions of the stone. On rotation of the plate, or, if possible, of the nicols together, the field passes from darkness to maximum brightness four times in a complete revolution, the relative angular intervals between these positions being right angles. These positions of darkness are known as the positions of extinction, and the plate is said to extinguish in them. This test is exceedingly delicate and reveals the double refraction even when the greatest difference in the refractive indices is too small to be measured directly.
Doubly refractive substances are of two kinds: uniaxial, in which there is one direction of single refraction, and biaxial, in which there are two such directions. In the case of the former the direction of one, the ordinary ray, is precisely the same as if the refraction were single, but the refractive index of the other ray varies from that of the ordinary ray to a second limiting value, the extraordinary refractive index, which may be either greater or less. If the extraordinary is greater than the ordinary refractive index the double refraction is said to be positive ; if less, to be negative. A biaxial substance is more complex. It possesses three principal directions, viz., the bisectrices of the directions of single refraction and the perpendicular to the plane containing them. The first two correspond to the greatest and least, and the last to the mean of the principal indices of refraction. If the acute bisectrix corresponds to the least refractive index, the double refraction is said to be positive, and if to the greatest, negative. The relation of the .direc-
GEM-STONES
tions of single refraction, s, to the three principal directions, a, b, c, is illustrated in Fig. 27 for the case of topaz, a positive mineral. The refractive indices of the rays traversing one -of the principal directions have the values corresponding to the other two. In the direction a we should measure the greatest and the mean of the principal refractive indices, in the direction b the greatest and the least, and in the direction c the mean and the least. The maximum amount of double refraction is there- fore in the direction b.
In the examination of a faceted stone, of the most usual shape, the simplest method is to lay the large facet, called the table, on a -b glass slip and view the stone through the small parallel facet, the culet. Should the
FIG. 27,-Relation of the latter not exist> * mav fre- two Directions of single quently happen that owing Refraction to the prin- to internal reflection no light emerges through the steeply inclined facets. This difficulty is easily overcome by immersing the stone in some highly refracting oil. A glass plate held by hand over the stone with a drop of the oil between it and the plate serves the purpose, and is perhaps a more convenient method. A stone which does not possess a pair of parallel facets should be viewed through any pair which are nearly parallel.
We have stated that a plate of glass has no effect on the field. Suppose, however, it were viewed when placed between the jaws of a tightened vice
DOUBLE REFRACTION 47
and thus thrown into a state of strain, it would then show double refraction, the amount of which would depend on the strain. Natural singly refractive substances frequently show phenomena of a similar kind. Thus diamond sometimes contains a drop of liquid carbonic acid, and the strain is revealed by the coloured rings surrounding the cavity which are seen when the stone is viewed between crossed nicols. Double refraction is also common in diamond even when there is no included matter to explain it, and is caused by the state of strain into which the mineral is thrown on release from the enormous pressure under which it was formed. Other minerals which display these so-called optical anomalies, such as fluor and garnet, are not really quite singly refractive at ordinary temperatures ; each crystal is composed of several double refractive individuals. But all such phenomena cannot be confused with the characters of minerals which ex- tinguish in the ordinary way, since the stone will extinguish in small patches and these will not be dark all at the same time ; further, the double re- fraction is small, and on revolving the stone between crossed nicols the extinction is not sharp. Paste stones are sometimes in a state of strain, and display slight, but general, double refraction. Hence the existence of double refraction does not necessarily prove that the stone is real and not an imitation. Stones may be composed of two or more individuals which are related to each other by twinning, in which case each individual would in general extinguish separately. Such individuals would be larger and would extinguish more sharply than the patches of an anomalous stone.
48
GEM-STONES
An examination in convergent light is sometimes of service. An auxiliary lens is placed over the condenser so as to converge the light on to the stone. Light now traverses the stone in different directions ; the more oblique the direction the greater the distance traversed in the stone. If it be doubly refractive, in any given direction there will be in general two rays with differing refractive indices and the resulting effect is akin to the well-known
phenomenon of New- ton's rings, and is an instance of what is termed interference. It may be mentioned that the interference of light (Fig. 28) explains such com- mon phenomena as the colours of a soap-bubble, the hues
of tarnished steel, the tints of a layer of oil floating on water, and so on. Light, after diverging from the stone, comes to focus a little beneath the plane in which the image of the stone is formed. An auxiliary lens must, therefore, be inserted to bring the focal planes together, so that the interference picture may be viewed by means of the same eye- piece.
If a uniaxial crystal be examined along the crystallographic axis in convergent light an inter- ference picture will be seen of the kind illustrated on Plate III. The arms of a black cross meet in the centre of the field, which is surrounded by a series of circular rings, coloured in white light. Rotation
FIG. 28.— Interference of Light.
I. UNIAXIAL
INTERFERENCE FIGURES
DOUBLE REFRACTION 49
of the stone about the axis produces no change in the picture.
A biaxial substance possesses two directions (the optic axes] along which a single beam is transmitted. If such a stone be examined along the line bisecting the acute angle between the optic axes (the acute bisectrix] an interference picture l will be seen which in particular positions of the stone with respect to the crossed nicols takes the forms illustrated on Plate III. As before, there is a series of rings which are coloured in white light ; they, however, are no longer circles but consist of curves known as lemniscates, of which the figure of 8 is a special form. Instead of an unchangeable cross there are a pair of black " brushes " which in one position of the stone are hyperbolae, and in that at right angles become a cross. On rotating the stone we find that the rings move with it and are unaltered in form, whereas the brushes revolve about two points, called the " eyes," where the optic axes emerge. If the observation were made along the obtuse bisectrix the angle between the optic axes would probably be too large for the brushes to come into the field, and the rings might not be visible in white light, though they would appear in monochromatic light. In the case of a substance like sphene the figure is not so simple, because the positions of the optic axes vary greatly for the different colours and the result is exceedingly complex ; in monochromatic light, however, the usual figure is visible.
It would probably not be possible in the case of
1 A cleavage flake of topaz may conveniently be used to show the phenomenon, but owing to the great width of the angle the "eyes" are invisible.
50 GEM-STONES
a faceted stone to find a pair of faces perpendicular to the required direction. Nevertheless, so long as a portion of the figures described is in the field of view, the character of the double refraction, whether uniaxial or biaxial, may readily be determined.
There is yet another remarkable phenomenon which must not be passed over. Certain substances, of which quartz is a conspicuous example and in this respect unique among the gem-stones, possess the remarkable property of rotating the plane of polarization of a ray of light which is transmitted parallel to the optic axis. If a plate of quartz be cut at right angles to the axis and placed between crossed nicols in white light, the field will be coloured, the hue changing on rotation of one nicol with respect to the other. Examination in monochromatic light shows that the field will become dark after a certain rotation of the one nicol with respect to the other, the amount of which depends on the thickness of the plate. If the plate be viewed in convergent light, an interference picture is seen as illustrated on Plate III, which is similar to, and yet differs in some important particulars from the ordinary interference picture of a uniaxial stone. The cross does not penetrate beyond the innermost ring and the centre of the field is coloured in white light. If a stone shows such a picture, it may be safely assumed to be quartz. It is interesting to note that minerals which possess this property have a spiral arrangement of the constituent atoms.
It has already been remarked (p. 28) that if a faceted doubly refractive stone be rotated with one facet always in contact with the dense glass of the refractometer the pair of shadow-edges that are
DOUBLE REFRACTION 51
visible in the field move up or down the scale in general from or to maximum and minimum positions. The manner in which this movement takes place depends upon the character of the double refraction and the position of the facet under observation with regard to the optical symmetry of the stone. In the case of a uniaxial stone, if the facet be perpendicular to the crystallographic axis, i.e. the direction of single refraction, neither of the shadow-edges will move. If the facet be parallel to that direction, one shadow-edge will move up and coincide with the other, which remains invariable in position, and away from it to a second critical position ; the latter gives the value of the extra- ordinary refractive index, and the invariable shadow- edge corresponds to the ordinary refractive index. This phenomenon is displayed by the table-facet 01 most tourmalines, because for reasons given above (p. 11) they are as a rule cut parallel to the crystallographic axis. In the case of facets in intermediate positions, the shadow-edge correspond- ing to the extraordinary refractive index moves, but not to coincidence with the invariable shadow-edge. The case of a biaxial stone is more complex. If the facet be perpendicular to one of the principal directions one shadow-edge remains invariable in position, corresponding to one of the principal refractive indices, whilst the other moves between the critical values corresponding to the remaining two of the principal refractive indices. In the interesting case in which the facet is parallel to the two directions of single refraction, the second shadow- edge moves across the one which is invariable in position. In intermediate positions of the facet both
52 GEM-STONES
shadow-edges move, and give therefore critical values. Of the intermediate pair, i.e. the lower maximum and the higher minimum, one corresponds to the mean principal refractive index, and the other depends upon the relation of the facet to the optical symmetry. If it is desired to distinguish between them, observations must be made on a second facet ; but for discriminative purposes such exactitude is unnecessary, since the least and the greatest refractive indices are all that are required.
The character of the refraction of gem-stones is given in Table V at the end of the book.
CHAPTER VII
ABSORPTION EFFECTS: COLOUR, DICHROISM, ETC.
WHEN white light passes through a cut stone, colour effects result which arise from a variety of causes. The most obvious is the funda- mental colour of the stone, which is due to its selective absorption of the light passing through it, and would characterize it before it was cut. Inter- mingled with the colour in a transparent stone is the dispersive effect known as 'fire,' which has already been discussed (p. 20). In many instances the want of homogeneity is responsible for some peculiar effects such as opalescence, chatoyancy, and asterism. These phenomena will now be considered in fuller detail.
COLOUR
All substances absorb light to some extent. If the action is slight and affects equally the whole of the visible spectrum, the stone appears white or colourless. Usually some portion is more strongly absorbed than the rest, and the stone seems to be coloured. What is the precise tint depends not only upon the portions transmitted through the stone, but also upon their relative intensities. The eye, unlike the ear, has not the power of analysis
54 GEM-STONES
and it cannot of itself determine how a composite colour has been made up. Indeed, so far as it is concerned, any colour may be exactly matched by compounding in certain proportions three simple primary colours — red, yellow, and violet. Alex- andrite, a variety of chrysoberyl, is a curious and instructive case. The balance in the spectrum of light transmitted through it is such that, whereas in daylight such stones appear green, in artificial light, especially in gas-light, they are a pronounced raspberry-red (Plate XXVII, Figs, n, 13). The phenomenon is intensified by the strong dichroism characteristic of this species.
The colour is the least reliable character that may be employed for the identification of a stone, since it varies considerably in the same species, and often results from the admixture of some metallic oxide, which has no essential part in the chemical com- position and is present in such minute quantities as to be almost imperceptible by analysis. Who would, for instance, imagine from their appearance that stones so markedly diverse in hue as ruby and sapphire were really varieties of the same species, corundum ? Again, quartz, in spite of the simplicity of its composition, displays extreme differences of tint. Nevertheless, certain varieties do possess a distinctive colour, emerald being the most striking example, and in other cases the trained eye can appreciate certain characteristic subtleties of shade. At any rate, the colour is the most obvious of the physical characters, and serves to provide a rough division of the species, and accordingly in Table II at the end of the book the gem-stones are arranged by their usual tints.
ABSORPTION EFFECTS
55
DlCHROISM
The two rays into which a doubly refractive stone splits up a ray of light are often differently absorbed by it, and in consequence appear on emergence differently coloured ; such stones are said to be dichroic. The most striking instance is a deep- brown tourmaline, which, except in very thin sections, is quite opaque to the ordinary ray. The light transmitted by a plate cut parallel to the
FIG. 29. — Dichroscope (actual size).
crystallographic axis is therefore plane-polarized ; before the invention by Nicol of the prism of Iceland- spar known by his name this was the ordinary method of obtaining light of this character (cf. p. 43). Again, in the case of kunzite and cordierite the difference in colour is so marked as to be obvious to the unaided eye ; but where the contrast is less pronounced we require the use of an instrument called a dichroscope, which enables the twin colours to be seen side by side.
Fig. 29 illustrates in section the construction of a dichroscope. The instrument consists essentially of
5 6 GEM-STONES
a rhomb of Iceland-spar, S, of such a length as to give two contiguous images (Fig. 30) of a square hole, //, in one end of the tube containing it. In some instruments the terminal faces of the rhomb are ground at right angles to its length, but usually, as in that depicted, prisms of glass, G, are cemented on to the two ends. A
CEP C> with a sli£htly lar£er h°le>
FIG. 30. -Field of ...... , ^
the Dichroscope. which is circular in shape, fits on the end of the tube, and can be moved up and down it and revolved round it, as desired. The stone, R, to be tested may be directly attached to it by means of some kind of wax or cement in such a way that light which has traversed it passes into the window, H, of the in- strument ; the cap at the same time permits of the rotation of the stone about the axis of the main tube of the instrument. The dichroscope shown in the figure has a still more convenient arrangement : it is provided with an additional attachment, A, by means of which the stone can be turned about an axis at right angles to the length of the tube, and thus examined in different directions. At the other end of the main tube is placed a lens, L, of low power for viewing the twin images : the short tube containing it can be pushed in and out for focusing purposes. Many makers now place the rhomb close to the lens, L, and thereby require a much smaller piece of spar ; material suitable for optical purposes is fast growing scarce.
Suppose that a plate of tourmaline cut parallel to its crystallographic axis is fastened to the cap and the latter rotated. We should notice, on looking
ABSORPTION EFFECTS 57
through the instrument, that in the course of a complete revolution there are two positions, ori- entated at right angles to one another, in which the tints of the two images are identical, the positions of greatest contrast of tint being midway between. If we examine a uniaxial stone in a direction at right angles to its optic axis we obtain the colours corresponding to the ordinary and the extraordinary rays. In any direction less inclined to the axis we still have the colour for the ordinary ray, but the other colour is intermediate in tint between it and that for the extraordinary ray. The phenomenon presented by a biaxial stone is more complex. There are three principal colours which are visible in differing pairs in the three principal optical directions ; in other directions the tints seen are intermediate between the principal colours. Since biaxial stones have three principal colours, they are sometimes said to be trichroic or pleochroic, but in any single direction they have two twin colours and show dichroism. No difference at all will be shown in directions in which a stone is singly refractive, and it is therefore always advisable to examine a stone in more than one direction lest the first happens to be one of single refraction. For determinative purposes it is __not ijiecessary to note the exact shades of tint of the twin colours, because they vary with the inherent colour of the stone, and are therefore not constant for the same species ; we need only observe, when the stone is tested with the dichroscope, whether there is any variation of colour, and, if so, its strength. Dichroism is a result of double refraction, and cannot exist in a singly refractive stone. The converse, however, is not true
58 GEM-STONES
and it by no means follows that, because no dichroism can be detected in a stone, it is singly refractive. A colourless stone, for instance, cannot possibly be dichroic, and many coloured, doubly refractive stones — for example, zircon — exhibit no dichroism, or so little that it is imperceptible. The character is always the better displayed, the deeper the inherent colour of the stone. The deep-green alexandrite, for instance, is far more dichroic than the lighter coloured varieties of chrysoberyl.
If the stone is attached to the cap of the instrument, the table should be turned towards it so as to assure that the light passing into the instru- ment has actually traversed the stone. If little light enters through the opposite coign, a drop of oil placed thereon will overcome the difficulty (cf. p. 46). It is also necessary, for reasons mentioned above, to examine the stone in directions as far as possible across the girdle also. A convenient, though not strictly accurate, method is to lay the stone with the table facet on a table and examine the light which has entered the stone and been reflected at that facet. The stone may easily be rotated on the table, and observations thus made in different directions in the stone. Care must be exercised in the case of a faceted stone not to mistake the alteration in colour due to dispersion for a dichroic effect, and the stone must be placed close to the instru- ment during an observation, because otherwise the twin rays traversing the instrument may have taken sensibly different directions in the stone.
Dichroism is an effective test in the case of ruby; its twin colours — purplish and yellowish red — are in marked contrast, and readily distinguish it from
ABSORPTION EFFECTS 59
other red stones. Again, one of the twin colours of sapphire is distinctly more yellowish than the other ; the blue spinel, of which a good many have been manufactured during recent years, is singly refractive, and, of course, shows no difference of tint in the dichroscope.
Table VI at the end of the book gives the strength of the dichroism of the gem-stones.
ABSORPTION SPECTRA
A study of the chromatic character of the light transmitted by a coloured stone is of no little interest. As was stated above, the eye has not the power of analysing light, and to resolve the trans- mitted rays into their component parts an instru- ment known as a spectroscope is needed. The small ' direct-vision ' type has ample dispersion for this purpose. It is advantageous to employ by preference the diffraction rather than the prism form, because in the former the intervals in the resulting spectrum corresponding to equal differences of wave-length are the same, whereas in the latter they diminish as the wave-length increases and accordingly the red end of the spectrum is relatively cramped.
The absorptive properties of all doubly refractive coloured substances vary more or less with the direction in which light traverses them according to the amount of dichroism that they possess, but the variation is not very noticeable unless the stone is highly dichroic. If the light transmitted by a deep- coloured ruby be examined with a spectroscope it will be found that the whole of the green portion
6o
GEM-STONES
of the spectrum is obliterated (Fig. 31), while in the case of a sapphire only a small portion of the red end of the spectrum is absorbed. Alexandrite affords especial interest. In the spectrum of the
ALMANDINE
ALEXANDRITE
RUBY
THE SOLAR SPECTRUM FIG. 31.— Absorption Spectra.
light transmitted by it, the violet and the yellow are more or less strongly absorbed, depending upon the direction in which the rays have passed through the stone (Fig. 31), and the transmitted light is mainly composed of two portions — red and green. The apparent colour of the stone depends, therefore, upon
ABSORPTION EFFECTS 61
which of the two predominates. In daylight the resultant colour is green flecked with red and orange, the three principal absorptive tints (cf. p. 235), but in artificial light, which is relatively stronger in the red portion of the spectrum, the resultant colour is a raspberry-red, and there is less apparent difference in the absorptive tints (cf. Plate XXVII, Figs. 1 1, 13).
In all the spectra just considered, and in all like them, the portions that are absorbed are wide, the passage from blackness to colour is gradual, and the edges deliminating them are blurred. In the spectra of certain zircons and in almandine garnet the absorbed portions, or bands as they are called, are narrow, and, moreover, the transition from black- ness to colour is sharp and abrupt ; such stones are therefore said to display absorption-bands. Church in 1866 was the first to notice the bands shown by zircon (Fig. 31). Sorby thought they portended the existence of a new element, to which he gave the name jargonium, but subsequently discovered that they were caused by the presence of a minute trace of uranium. A yellowish-green zircon shows the phenomenon best, and it has all the bands shown in the figure. The spectrum varies slightly but almost imperceptibly with the direction in the stone. Others show the bands in the yellow and green, while others show only those in the red, and some only one of them. The bands are not confined to stones of any particular colour, or amount of double refraction. Again, many zircons show no bands at all, so that their absence by no means precludes the stone from being a zircon.
Almandine is characterized by a different spectrum (Fig. 31). The band in the yellow is the most con-
62 GEM-STONES
spicuous, and is no doubt responsible for the purple hue of a typical almandine. The spectrum varies in strength in different stones. Rhodolite (p. 2 14), a garnet lying between almandine and pyrope, displays the same bands, and indications of them may be detected in the spectra of pyropes of high refraction.
JEWELLERY DESIGNS
64 GEM-STONES
species, and is therefore very useful for discrimina- tive purposes. It can be determined whatever be the shape of the stone, and it is immaterial whether it be transparent or not ; but, on the other hand, the stone must be unmounted and free from the setting. The methods for the determination of the specific gravity are of two kinds : in the first a liquid is found of the same, or nearly the same, density as the stone, and in the second weighings are made and the use of an accurate balance is required.
(i) HEAVY LIQUIDS
Experiment tells us that a solid substance floats in a liquid denser than itself, sinks in one less dense, and remains suspended at any level in one of pre- cisely the same density. If the stone be only slightly less dense than the liquid, it will rise to the surface ; if it be just as slightly denser, it will as surely sink to the bottom, a physical fact which has added so much to the difficulty and danger of sub- marine manoeuvring. If then we can find a liquid denser than the stone to be tested, and place the latter in it, the stone will float on the surface. If we take a liquid which is less dense than the stone and capable of mixing with the heavier liquid, and add it to the latter, drop by drop, gently stirring so as to assure that the density of the combination is uniformly the same throughout, a stage is finally reached when the stone begins to move downwards. It has now very nearly the density of the liquid, and, if we find by some means this density, we know simultaneously the specific gravity of the stone.
SPECIFIC GRAVITY 65
Various devices and methods are available for ascertaining the density of liquids — for instance, Westphal's balance ; but, apart from the incon- venience attending such a determination, the density of all liquids is somewhat seriously affected by changes in the temperature, and it is therefore better to make direct comparison with fragments of sub- stances of known specific gravity, which are termed indicators. If of two fragments differing slightly in specific gravity one floats on the surface of a uniform column of liquid and the other lies at the bottom of the tube containing the liquid, we may be certain that the density of the liquid is intermediate between the two specific gravities. Such a precaution is necessary because, if the liquid be a mixture of two distinct liquids, the density would tend to increase owing to the greater volatility of the lighter of them, and in any case the density is affected by change of temperature. The specific gravity of stones is not much altered by variation in the temperature.
A more convenient variation of this method is to form a diffusion column, so that the density increases progressively with the depth. If the stone under test floats at a certain level in such a column inter- mediate between two fragments of known specific gravity, its specific gravity may be found by elementary interpolation. To form a column of this kind the lighter liquid should be poured on to the top of the heavier. Natural diffusion gives the most perfect column, but, being a lengthy process, it may conveniently be quickened by gently shaking the tube, and the column thus formed gives results sufficiently accurate for discriminative purposes. 5
66 GEM-STONES
By far the most convenient liquid for ordinary use is methylene iodide, which has already been recommended for its high refraction. It has, when pure, a density at ordinary room-temperatures of 3'324, and it is miscible in all proportions with benzol, whose density is cr88, or toluol, another hydrocarbon which is somewhat less volatile than benzol, and whose density is about the same, namely, O'86. When fresh, methylene iodide has only a slight tinge of yellow, but it rapidly darkens on exposure to light owing to the liberation of iodine which is in a colloidal form and cannot be removed by filtration, The liquid may, however, be easily cleared by shaking it up with any substance with which the iodine combines to form an iodide remov- able by filtration. Copper filings answer the purpose well, though rather slow in action ; mercury may also be used, but is not very satisfactory, because a small amount may be dissolved and afterwards be precipitated on to the stone under test, carrying it down to the bottom of the tube. Caustic potash (potassium hydroxide) is also recommended ; in this case the operation should preferably be carried out in a special apparatus which permits the clear liquid to be drawn off underneath, because water separates out and floats on the surface. In Fig. 32 three cut stones, a quartz (ft), a beryl (£), and a tourmaline (c) are shown floating in a diffusion column of methy- lene iodide and benzol. Although the beryl is only slightly denser than the quartz, it floats at a perceptibly lower level. These three species are occasionally found as yellow stones of very similar tint.
Various other liquids have been used or proposed
SPECIFIC GRAVITY
for the same purpose, of which two may be mentioned. The first of them is a saturated solu- tion of potassium iodide and mercuric iodide in water, which is known after the discoverer as Sonstadt's solution. It is a clear mobile liquid with an amber colour, having at 12° C. a density of 3*085 ; it may be mixed with water to any extent, and is easily concentrated by heating; moreover, it is durable and not sub- ject to alteration of any kind ; on the other hand, it is highly poisonous and cauterizes the skin, not being checked by albumen ; it also de-
stroys brass-ware by amalgamating FlG> 32._Stones
the metal. The second is Klein's of different Spe-
solution, a clear yellow liquid which cific Gravities
has at 1 5' C. a density of 3-28. It S?£2
Consists of the boro - tungState Of of heavy Liquid.
cadmium, of which the formula is 9WO3.B2O3.2CdO.2H2O+i6Aq, dissolved in water, with which it may be diluted. If the salt be heated, it fuses at 75° C. in its own water of crystallization to a yellow liquid, very mobile, with a density of 3-55. Klein's solution is harmless, but it cannot compare for convenience of manipulation with methylene iodide.
The most convenient procedure is to have at hand three glass tubes, fitted with stoppers or corks, to contain liquids of different densities —
(a) Methylene iodide reduced to 27 ; using as indicators orthoclase 2*55, quartz 2*66, and beryl 274.
68 GEM-STONES
(£) Methylene iodide reduced to 3-1 ; indicators, beryl 2*74 and tourmaline 3*10.
(c) Methylene iodide, undiluted, 3'32.
The pure liquid in the last tube should on no account be diluted ; but the density of the other two liquids may be varied slightly, either by adding benzol in order to lower it, or by allowing benzol, which has far greater volatility than methylene iodide, to evaporate, or by adding methylene iodide, in order to increase it. The density of the liquids may be ascertained approximately from the in- dicators.
A glance at the table of specific gravities shows that as regards the gem-stones methylene iodide is restricted in its application, since it can be used to test only moonstone, quartz, beryl, tourmaline, and spodumene; opal and turquoise, being amorphous and more or less porous, should not be immersed in liquids, lest the appearance of the stone be irre- trievably injured. Methylene iodide readily serves to distinguish the yellow quartz from the true topaz, with which jewellers often confuse it, the latter stone sinking in the liquid ; again aquamarine floats, but the blue topaz, which is often very similar to it, sinks in methylene iodide.
By saturating methylene iodide with iodine and iodoform, we have a liquid (d} of density 3'6 ; a fragment of topaz, 3-55, may be used to indicate whether the liquid has the requisite density. Un- fortunately this saturated solution is so dark as to be almost opaque, and is, moreover, very viscous. Its principal use is to distinguish diamond, 3'535, from the brilliant colourless zircon, with which, apart from a test for hardness, it may easily be
SPECIFIC GRAVITY 69
confused. It is easy to see whether the stone floats, as it would do if a diamond. To recover a stone which has sunk, the only course is to pour off the liquid into another tube, because it is far too dark for the position of the stone to be seen.
It is possible to employ a similar method for still denser stones by having recourse to Retgers's salt, silver-thallium nitrate. This double salt is solid at ordinary room-temperatures, but has the remarkable property of melting at a temperature, 75° C., which is well below the point of fusion of either of its constituents, to a clear, mobile yellow liquid, which is miscible in any proportion with water, and has, when pure, a density of 4/6. The salt may be purchased, or it may be prepared by mixing 100 grams of thallium nitrate and 64 grams of silver nitrate, or similar proportions, in a little water, and heating the whole over a water-bath, keeping it constantly stirred with a glass rod until it is liquefied. The two salts must be mixed in the correct proportions, because otherwise the mixture might form other double salts, which do not melt at so low a temperature. A glance at the table of specific gravities shows that Retgers's salt may be used for all the gem-stones with the single exception of zircon (b). There are, however, some objections to its use. It is expensive, and, unless kept con- stantly melted, it is not immediately available. It darkens on exposure to strong sunlight like all silver salts, stains the skin a peculiar shade of purple which is with difficulty removed, and in fact only by abrasion of the skin, and, like all thallium compounds, is highly poisonous.
It is convenient to have three tubes, fitted as
70 GEM-STONES
before with stoppers or corks, to contain the follow- ing liquids, when heated : —
(e) Silver-thallium nitrate, reduced to 3'5 ; using
as indicators, peridot or idocrase 3-40 and topaz
3*53.
(/) Silver-thallium nitrate, reduced to 4-0; in- dicators, topaz 3-53 and sapphire 4-03.
(g) Silver-thallium nitrate, undiluted, 4*6.
The tubes must be heated in some form of water- bath ; an ordinary glass beaker serves the purpose satisfactorily. The pure salt should never be diluted; but the density of the contents of tubes (e) and (/) may be varied at will, water being added in order to lower the density, and concentra- tion by means of evaporation or addition of the nitrate being employed in order to increase it. To avoid the discoloration of the skin, rubber finger- stalls may be used, and the stones should not be handled until after they have been washed in warm water. The staining may be minimized if the hands be well washed in hot water before being exposed to sunlight. It is advisable to warm the stone to be tested in a tube containing water be- forehand lest the sudden heating develop cracks. A piece of platinum, or, failing that, copper wire is of service for removing stones from the tubes ; a glass rod, spoon-shaped at one end, does equally well. It must be noted that although Retgers's salt is absolutely harmless to the ordinary gem- stones — with the exception of opal and turquoise, which, as has already been stated, being to some extent porous, should not be immersed in liquids — it attacks certain substances, for instance, sulphides and cannot be applied indiscriminately to minerals.
SPECIFIC GRAVITY 71
The procedure described above is intended only as a suggestion ; the method may be varied to any extent at will, depending upon the particular re- quirements. If such tests are made only occasion- ally, a smaller number of tubes may be used. Thus one tube may be substituted for the two marked a and b, the liquid contained in it being diluted as required, and a series of indicators may be kept apart in small glass tubes. On the other hand, any one having constantly to test stones might in- crease the number of tubes with advantage, and might find it useful to have at hand fragments of all the principal species in order to make direct comparison.
(2) DIRECT WEIGHING
The balance which is necessary in both the methods described under this head should be capable of giving results accurate to milligrams, i.e. the thousandth part of a gram, and con- sistent with that restriction the beam may be as short as possible so as to give rapid swings and thus shorten the time taken in the observations. A good assay balance answers the purpose admirably. Of course, it is never necessary to wait till the balance has come to rest. The mean of the extreme readings of the pointer attached to the beam will give the position in which it would ultimately come to rest. Thus, if the pointer just touches the eighth division on the right-hand side and the second on the other, the mean position is the third division on the right-hand side (|(8 — 2) = 3). Instead of the ordinary form of chemical balance, Westphal's form or Joly's spring-balance
72 GEM-STONES
may be employed. Weighings are made more quickly, but are not so accurate.
In refined physical work the practice known as double-weighing is employed to obviate any slight error there may be in the suspension of the balance. A counterpoise which is heavier than anything to be weighed is placed in one pan, and weighed. The counterpoise is retained in its pan throughout the whole course of the weighings. Any substance whose weight is to be found is placed in the other pan, and weights added till the balance swings truly again. The difference between the two sets of weights evidently gives the weight of the sub- stance. Balances, however, are so accurately con- structed that for testing purposes such refined precautions are not really necessary.
It is immaterial in what notation the weighings are made, so long as the same is used throughout, but the metric system of weights, which is in universal use in scientific work, should preferably be employed. Jewellers, however, use carat weights, and a subdivision to the base 2 instead of decimals, the fractions being £, £-, £, ^ J& -fa- If these weights be employed, it will be necessary to convert these fractions into decimals, and write | = '5oo,
i = -250, i = -i 25, TV = -062, ^= -03 1, ^ = -016.
(a) Hydrostatic Weighing
The principle of this method is very simple. The stone, the specific gravity of which is required, is first weighed in air and then when immersed in water. If W and W be these weights respectively, then W —W is evidently the weight of the water
SPECIFIC GRAVITY 73
displaced by the stone and having therefore the same volume as it, and the specific gravity is there-
W
fore equal to w _ w/-
If the method of double-weighing had been adopted, the formula would be slightly altered. Thus, suppose that c corresponds to the counter- poise, w and w' to the stone weighed in air and water respectively ; then we have W — c — w and
FIG. 33.— Hydrostatic Balance.
W' = c — w'y and therefore the specific gravity is
c - w equal to — -. .
w - w
Some precautions are necessary in practice to assure an accurate result A balance intended for specific gravity work is provided with an auxiliary pan (Fig. 33), which hangs high enough up to permit of the stone being suspended underneath. The weight of anything used for the suspension must, of course, be determined and subtracted from the weight found for the stone, both when in air and when in water. A piece of fine silk is generally
74 GEM-STONES
used for suspending the stone in water, but it should be avoided, because the water tends to creep up it and the error thus introduced affects the first place of decimals in the case of a one-carat stone, the value being too high. A piece of brass wire shaped into a cage is much to be preferred. If the same cage be habitually used, its weight in air and when immersed in water to the customary extent in such determinations should be found once for all.
Care must also be taken to remove all air-bubbles which cling to the stone or the cage ; their presence would tend to make the value too low. The surface tension of water which makes it cling to the wire prevents the balance swinging freely, and renders it difficult to obtain a weighing correct to a milligram when the wire dips into water. This difficulty may be overcome by substituting a liquid such as toluol, which has a much smaller surface tension.
As has been stated above, the density of water at 4° C. is taken as unity, and it is therefore necessary to multiply the values obtained by the density of the liquid, whatever it be, at the tempera- ture of the observation. In Table IX, at the end of the book, are given the densities of water and toluol at ordinary room-temperatures. It will be noticed that a correct reading of the temperature is far more important in the case of toluol.
Example of a Hydrostatic Determination of Specific Gravity —
Weight of stone in air = I '47 1 gram Weight of stone in water = I '067 ,,
SPECIFIC GRAVITY 75
Allowing for the density of water at the tempera- ture of the room, which was 16° C., the specific gravity is 3'637. Had no such allowance been made, the result would have been four units too high in the third place of decimals. For discriminative purposes, however, such refinement is unnecessary.
(b) Pycnometer^ or Specific Gravity Bottle
The specific gravity bottle is merely one with a fairly long neck on which a horizontal mark has been scratched, and which is closed by a ground glass stopper. The pycnometer is a refined variety of the specific gravity bottle. It has two openings : the larger is intended for the insertion of the stone and the water, and is closed by a stopper through which a thermometer passes, while the other, which is exceedingly narrow, is closed by a stopper fitting on the outside, and is graduated to facilitate the determination of the height of the water in it.
The stone is weighed as in the previous method. The bottle is then weighed, and filled with water up to the mark and weighed again. The stone is now introduced into the bottle, and the surplus water removed with blotting-paper or otherwise until it is at the same level as before, and the bottle with its contents is weighed. Let W be the weight of the stone, w the weight of the bottle, W the weight of the bottle and the water contained in it, and W" the weight of the bottle when containing the stone and the water. Then W -w is the weight of the water filling the bottle up to the mark, and W" — w — W is the reduced weight of water after the stone has been inserted ; the difference,
76 GEM-STONES
W+W- W"t is the weight of the water displaced.
W
The specific gravity is therefore - — — 5.
W + W — W
As in the previous method, this value must be multiplied by the density of the liquid at the temperature of the experiment. If the method of double-weighing be adopted, the formula will be slightly modified.
Of the above methods, that of heavy liquids, as it is usually termed, is by far the quickest and the most convenient for stones of ordinary size, the specific gravity of which is less than the density of pure methylene iodide, namely, 3*324, and by its aid a value may be obtained which is accurate to the second place of decimals, a result quite sufficient for a discriminative test. The method is applicable no matter how small the stone may be, and, indeed, for very small stones it is the only trustworthy method ; for large stones it is inconvenient, not only because of the large quantity of liquid required, but also on account of the difficulty in estimating with sufficient certainty the position of the centre of gravity of the stone. A negative determination may be of value, especially if attention be paid to the rate at which the stone falls through the liquid ; the denser the stone the faster it will sink, but the rate depends also upon the shape of the stone. Retgers's salt is less convenient because of the delay involved in warming it and of the almost inevitable staining of the hands, but its use presents no difficulty whatever.
Hydrostatic weighing is always available, unless the stone be very small, but the necessary weighings
SPECIFIC GRAVITY 77
occupy considerable time, and care must be taken that no error creeps into the computation, simple though it be. Even if everything is at hand, a determination is scarcely possible under a quarter of an hour.
The third method, which takes even longer, is intended primarily for powdered substances, and is not recommended for cut stones, unless there happen to be a number of tiny ones which are known to be exactly of the same kind.
The specific gravities of the gem-stones are given in Table VII at the end of the book.
CHAPTER IX HARDNESS AND CLE A V ABILITY
EVERY possessor of a diamond ring is aware that diamond easily scratches window-glass. If other stones were tried, it would be found that they also scratched glass, but not so readily, and, if the experiment were extended, it would be found that topaz scratches quartz, but is scratched by corundum, which in its turn yields to the all- powerful diamond. There is therefore considerable variation in the capacity of precious stones to resist abrasion, or, as it is usually termed, in their hardness. To simplify the mode of expressing this character the mineralogist Mohs about a century ago devised the following arbitrary scale, which is still in general use.
MOHS'S SCALE OF HARDNESS
|
i. Talc 2. Gypsum 3. Calcite |
4. Fluor 5. Apatite 6. Orthoclase 10. Diamond |
7. Quartz 8. Topaz 9. Corundum |
A finger-nail scratches gypsum and softer sub- stances. Ordinary window-glass is slightly softer than orthoclase, and a steel knife is slightly harder ;
HARDNESS AND CLEAV ABILITY 79
a hardened file approaches quartz in hardness, and easily scratches glass.
By saying that a stone has hardness 7 we merely mean that it will not scratch quartz, and quartz will not scratch it. The numbers indicate an order, and have no quantitative significance whatever. This is an important point about which mistakes are often made. We must not, for instance, suppose that diamond has twice the hardness of apatite. As a matter of fact, the interval between diamond and corundum is immensely greater than that between the latter and talc, the softest of mineral substances. Intermediate degrees of hardness are expressed by fractions. The number 8£ for chrysoberyl means that it scratches topaz as easily as it itself is scratched by corundum. Pyrope garnet is slightly harder than quartz, and its hardness is said therefore to be "j\>
Delicate tests show that the structure of all crystallized substances is more or less grained, like that of wood, and the hardness for the same stone varies in different directions. Kyanite is unique in this respect, since its hardness ranges from 5 to 7 ; it can therefore be scratched by a knife in some directions, but not in others. In most substances, however, the range is so small as to be quite imper- ceptible. Slight variation is also apparent in the hardness of different specimens of the same species. The diamonds from Borneo and New South Wales are so distinctly harder than those from South Africa and other localities that, when first discovered, some difficulty was experienced in cutting them. Again, lapidaries find that while Ceylon sapphires are harder than rubies, Kashmir sapphires are softer.
8o GEM-STONES
Hardness is a character of fundamental importance in a stone intended for ornamental wear, since upon it depends the durability of the polish and brilliancy. Ordinary dust is largely composed of grains of sand, which is quartz in a minute form, and a gem-stone should therefore be at least as hard as that Paste imitations are little harder than 5, and consequently, as experience shows, their polish does not survive a few weeks' wear. Hardness is, however, of little use as a discriminative test except for distinguishing between topaz or harder stone and paste. Diamond is so much harder than other stones that it will leave a cut in glass quite different from the scratch of even corundum. Paste, being so soft, readily yields to the file, and is thus easily distinguished from genuine stones. In applying the test to a cut stone, it is best to remove it from its mount and try the effect on the girdle, because any scratch would be concealed afterwards by the setting. Any mark should be rubbed with the finger to assure that it is not due to powder from the scratching agent ; confusion may often be caused in this way when the two substances are of nearly the same hardness.
The degrees of hardness of the gem-stones are given in Table VIII at the end of the book.
It must not be overlooked that extreme hardness is compatible with cleavability in certain directions intimately connected with the crystalline structure ; the property, in fact, characterizes many mineral species of different degrees of hardness. Diamond can be split in four directions parallel to the faces of the regular octahedron, a property utilized by the
HARDNESS AND CLEAVABILITY 81
lapidary for shaping a stone previous to cutting it. Topaz cleaves with considerable ease at right angles to the principal crystallographic axis. Felspar has two directions of cleavage nearly at right angles to one another. The new gem-stone, kunzite, needs cautious handling owing to the facility with which it splits in two directions mutually inclined at about 70°.
All stones are more or less brittle, and will be fractured by a sufficiently violent blow, but the irregular surface of a fracture cannot be mistaken for the brilliant flat surface given by a cleavage. The cleavage is by no means induced with equal facility in the species mentioned above. A consider- able effort is required to split diamond, but in the case of topaz or kunzite incipient cleavage in the shape of flaws may be started if the stone be merely dropped on to a hard floor.
CHAPTER X ELECTRICAL CHARACTERS
THE definite orientation of the molecular arrangement of crystallized substances leads in many cases to attributes which vary with the direction and are revealed by the electrical properties. If a tourmaline crystal be heated in a gas or alcohol flame it becomes charged with electricity, and, since it is at the same time a bad conductor, static charges of opposite sign appear at the two ends. Topaz shows similar characters, but in a lesser degree. Quartz, if treated in the same way, shows charges of opposite sign on different sides, but the phenomenon may be masked by intimate twinning and consequent overlapping of the contrary areas. The phenomenon may also be seen when the stones are cut. The most convenient method for detecting the existence of the electrical charges is that devised by Kundt A powder consisting of a mixture of red lead and sulphur is placed in a bellows arrangement and blown through a sieve at one end on to the stone. Owing to the friction the particles become electrified — red lead positively and sulphur negatively — and are attracted by the charges of opposing sign, which will therefore be betrayed by the colour of the dust at the corre- sponding spot. The powder must be kept dry ;
ELECTRICAL CHARACTERS 83
otherwise a chemical reaction may occur leading to the formation of lead sulphide, recognizable by its black colour. Bucker has suggested as an alterna- tive the use of sulphur, coloured red with carmine, the negative element, and yellow lycopodium, the positive element.
Diamond, topaz, and tourmaline are powerful enough, when electrified by friction with a cloth, to attract fragments of paper, the electrification being positive. Amber develops considerable negative electricity when treated in a similar manner.
Diamond is translucent to the Rontgen (X) rays ; glass, on the other hand, is opaque to them, and this test distinguishes brilliants from paste imitations. Diamond also, unlike glass, phosphoresces under the influence of radium, a property characterizing also kunzite.
It will be seen that the electrical characters, although of considerable interest to the student, are, on account of their limited application and difficulty of test, of little service for the discrimination of gem-stones.
PART I— SECTION B
THE TECHNOLOGY OF GEM- STONES
CHAPTER XI UNIT OF WEIGHT
THE system in use for recording the weights of precious stones is peculiar to jewellery. The unit, which is known as the carat, bears no simple relation to any unit that has existed among European nations, and indubitably has been intro- duced from the East. When man in early days sought to record the weights of small objects, he made use of the most convenient seeds or grains which were easily obtainable and were at the same time nearly uniform in size. In Europe the smallest unit of weight was the barley grain. Similarly in the East the seeds of some leguminous tree were selected. Those of the locust-tree, Cera- tonia siliqua, which is common in the countries bordering the Mediterranean, on the average weigh so nearly a carat that they almost certainly formed the original unit. It is, indeed, from the Greek Kepdnov, little horn, which refers to the shape of the pods, that the word carat is derived.
UNIT OF WEIGHT 85
It is one of the eccentricities of the jewellery trade that precision should not have been given to the unit of weight. Not only does it vary at most of the trade centres in the world, but it is not even always constant at each centre. The difference is negligible in the case of single stones of ordinary size, but becomes a matter of serious importance when large stones, or parcels of small stones, are bought and sold, particularly when the stones are very costly. Attempts have been made at various times to secure a uniform standard, but as yet with only partial success. In 1871 the carat defined as the equivalent of 0*20500 gram was suggested at a meeting of the principal jewellers of Paris and London, and was eventually accepted in Paris, New York, Leipzig, and Borneo. It has, however, recently been recognized that in view of the gradual spread of the metric system of weights and measures the most satisfactory unit is the metric carat of one-fifth (0*2) gram. This has now been constituted the legal carat of France and Belgium, and no doubt other countries will follow their example. The carat weight obtaining in London weighs about 0-20530 gram, and the approximate equivalents in the gram at other centres are as follows: — Florence 0-19720, Madrid 0*20539, Berlin 0*20544, Amsterdam 0*20570, Lisbon 0*20575, Frankfort - on - Main 0*20577, Vienna 0-20613, Venice 0*20700, and Madras 0*20735. The gram itself is inconveniently large to serve as a unit for the generality of stones met with in ordinary jewellery.
The notation for expressing the sub-multiples of the carat forms another curious eccentricity.
86 GEM-STONES
Fractions are used which are powers of the half: thus the half, the half of that, i.e. the quarter, and so on down to the sixty-fourth, and the weight of a stone is expressed by a series of fractions, e.g. SaieV carats. In the case of diamond a single unreduced fraction to the base 64 is substituted in place of the series of single fractions, and the weight of a stone is stated thus, 4|-£ carats. With the introduction of the metric carat the more con- venient and rational decimal notation would, of course, be simultaneously adopted.
Figs. 34-39 illustrate the exact sizes of diamonds
10 carats. FIGS. 34-39.— Exact Sizes of Brilliants of various Weights.
of certain weights, when cut as brilliants. The sizes of other stones depends upon their specific gravity, the weight varying as the volume multiplied by the specific gravity. Quartz, for instance, has a low specific gravity and would be perceptibly larger, weight for weight; zircon, on the other hand, would be smaller.
It has been found more convenient to select a smaller unit in the case of pearls, namely, the pearl-grain, four of which go to the carat.
Stencil gauges are in use for measuring approxi- mately the weight in carats of diamond brilliants and of pearls, which in both instances must be unmounted. A more accurate method for determining the weight
UNIT OF WEIGHT 87
of diamonds has been devised by Charles Moe, which is applicable to either unmounted or mounted stones. By means of callipers, which read to three-tenths of a millimetre, the diameter and the depth of the stone are measured, and by reference to a table the corre- sponding weight is found ; allowance is made for the varying fineness of the girdle, and, in the case of large stones, for the variation from a strictly circular section.
Since this chapter was written the movement in favour of the metric carat has made rapid progress, and this unit will soon have been adopted as the legal standard all over the world, even in countries, such as the British Isles and the United States, where the metric system is not in use. The advantage of an international unit is too obvious to need arguing.
CHAPTER XII FASHIONING OF GEM-STONES
ALTHOUGH many of the gem-stones have been endowed by nature with brilliant lustrous faces and display scintillating reflections from their surfaces, yet their form is never such as to reveal to full perfection the optical qualities upon which their charm depends. Moreover, the natural faces are seldom perfect ; as a rule the stones are broken either through some convulsion of the earth's crust or in course of extraction from the matrix in which they have lain, or they are roughened by attrition against matter of greater hardness, or worn by the prolonged action of water, or etched by solvents. Beautiful octahedra of diamond or spinel have been mounted without further embellishment, but even their appearance might have been much improved at the lapidary's hands.
By far the oldest of the existing styles of cutting is the rounded shape known as cabochon, a French word derived from the Latin cabo, a head. In the days of the Roman Empire the softer stones were often treated in this manner ; such stones were supposed to be beneficial to those suffering from short-sightedness, the reason no doubt being that transparent stones when cut as a double cabochon formed a convex lens. According to Pliny, Nero
JEWELLERY DESIGNS
FASHIONING OF GEM-STONES 89
had an emerald thus cut, through which he was accustomed to view the gladiatorial shows. This style of cutting was long a favourite for coloured stones, such as emerald, ruby, sapphire, and garnet, but has been abandoned in modern practice except for opaque, semi-opaque, and imperfect stones. The crimson garnet, which was at one time known by the name carbuncle, was so systematically thus cut that the word has come to signify a red garnet of this form. It was a popular brooch-stone with our grandmothers, but is no longer in vogue. The East still retains a taste for stones cut in the form of beads and drilled through the centre; the beads are threaded together, and worn as necklaces. The native lapidaries often improve the colour of pale emeralds by lining the hole with
. / FIG. 40.— Double
green paint. (Convex) Ca.
The cabochon form may be of bochon. three different kinds. In the first, the double cabochon (Fig. 40), both the upper and the under sides of the stones are curved. The curvature, however, need not be the same in each case; indeed, it is usually markedly different Moonstones and starstones are generally cut very steep above and shallow underneath. Occasion- ally a ruby or a sapphire is, when cut in this way, set with the shallow side above, because the light that has penetrated into the stone from above is more wholly reflected from a steep surface with consequent increase in the glow of colour from the stone. Opals are always cut higher on the exposed side, but the slope of the surface varies considerably ; they are generally cut steeply when required for
90 GEM-STONES
mounting in rings. Chrysoberyl cat's-eyes are invariably cut with curved bases in order to preserve the weight as great as possible. The double cabochon form with a shallow surface underneath merges into the second kind (Fig. 41) in which the under side is plane, the form commonly employed for quartz cat's-eyes, and occasion- ally also for carbuncles. In this type
the plane side is invariably mounted FIG. 41. — Simple r J
Cabochon. down wards. In the third form (Fig. 42) the curvature of the under surface is reversed, and the stone is hollowed out into a concave shape. This style is reserved for dark stones, such as carbuncles, which, if cut at all thick, would show very little colour. A piece of foil is often placed in the hollow in order to increase the reflection of light, and thus to heighten the colour effect. .^^SS^^
In early days it was supposed that
, , , f ,. FIG. 42.— Double
the extreme hardness of diamond (Concavo - con- precluded the possibility of fashion- vex) Cabochon. ing it, and up to the fifteenth century all that was done was to remove the gum-like skin which disfigured the Indian stones and to polish the natural facets. The first notable advance was made in 1475, when Louis de Berquem discovered, as it is said quite by accident, that two diamonds if rubbed together ground each other. With confident courage he essayed the new art upon three large stones entrusted to him by Charles the Bold, to the entire satisfaction of his patron. The use of wheels or discs charged with diamond dust soon followed, but at first the lapidaries evinced their victory over such stubborn material by grinding diamond into
FASHIONING OF GEM-STONES 91
divers fantastic shapes, and failed to realize how much might be done to enhance the intrinsic beauty of the stones by the means now at their disposal The Indian lapidaries arrived at the same discovery independently, and Tavemier found, when visiting the country in 1665, a large number of diamond cutters actively employed. If the stone were perfectly clear, they con- tented themselves with polishing the natural facets ; but if it contained flaws or specks, they covered it with numerous small facets haphazardly placed. The stone was invariably left in almost its original shape, and no effort was made to improve the symmetry.
For a long time little further progress wasmade, and even nearly a century after Berquem the only regular patterns known to Kentmann, who wrote in 1 562, were the diamond-point and the diamond-table (Figs. 43—44). The former consisted of the natural
octahedron facets ground to regular
/ \ shape, and was long employed for
~~x the minute stones which were set
in conjunction with large coloured
FIG —Table s^ones *n rings. The table repre-
Cut (side view), sented considerably greater labour
One corner of the regular octahedron was ground down until the artificial facet thus pro- duced was half the width of the stone, while the opposite corner was slightly ground.
Still another century elapsed before the introduc- tion of the rose pattern, which comprised twenty- four triangular facets and a flat base (Figs. 45—46), the stone being nearly hemispherical in shape. This
92 GEM-STONES
style is said to have been the invention of Cardinal Mazarin, but probably he was the first to have diamonds of any considerable size cut in this form. At the present day only tiny stones are cut as roses.
A few more years passed away, and at length at the close of the seventeenth century diamond came by its own when Vincenzio Peruzzi, a Venetian, introduced the brilliant form of cutting, and revealed for the first time its amazing c fire.' Except for minor changes this form remains
,FiG.45.-RoseCut tO this day the standard style for
(top view). the shape of diamond, and the word
brilliant is commonly employed to
denote diamond cut in this way. So obviously and
markedly superior is the style to all others that
upon its discovery the owners of large roses had
them re-cut as brilliants despite the loss in weight
necessitated by the change.
The brilliant form is derived from the old table by increasing the number of facets and slightly altering the angles pertaining to the natural octa- hedron. In a perfect brilliant FlG 46<_Rose Cut (Figs. 47-49) there are altogether '(side view). 58 facets, 33 above and 25 below the girdle, as the edge separating the upper and lower portions of the stone is termed, which are arranged in the following manner. Eight star- facets, triangular in shape, immediately surround the large table-facet. Next come four large templets or bezels, quadrilateral in form, arranged in pairs on opposite sides of the table-facet, the
FASHIONING OF GEM-STONES
93
four quoins or lozenges, similar in shape, coming intermediately between them ; in modern practice, however, these two sets are identical in shape and size, and there are consequently eight facets of the same kind instead of two sets of four. The eight
FlG. 47.— Brilliant Cut (top view).
FIG. 48.— Brilliant Cut (base view).
cross or skew facets and the eight skill facets, in both sets the shape being triangular, form the boundary of the girdle ; modern brilliants usually have instead sixteen facets of the same shape and size. The above 33 facets lie above the girdle and form the crown of the stone. Imme- diately opposite and parallel to the table is the tiny culet. Next to the latter come the four large pavilion facets with the four quoins intermediately between them, both sets being five-sided but nearly quadrilateral in shape ; these again are usually com- bined into eight facets of the same size. Eight cross facets and eight skill facets, both sets, like those in the crown, being triangular in shape, form the lower side of the girdle ; these also are generally united into a set of sixteen similar facets. These 2 5 facets which lie below the girdle comprise the ' pavilion,'
FIG. 49.— Brilliant Cut (side view).
94 GEM-STONES
or base of the stone. In a regular stone properly cut a templet is nearly parallel to a pavilion, and an upper to a lower cross facet. The contour of the girdle is usually circular, but occasionally assumes less symmetrical shapes, as for instance in drop- stones or pendeloques, and the facets are at the same time distorted. The number of facets may with advantage be increased in the case of large stones. An additional set of eight star facets is often placed round the culet, the total number then being 66. It may be mentioned that the largest stone cut from the Cullinan has the exceptional number of 74 facets.
In order to secure the finest optical effect certain proportions have been found necessary. The depth of the crown must be one-half that of the base, and therefore one-third the total depth of the stone, and the width of the table must be slightly less than half that of the stone. The culet should be quite small, not more in width than one-sixth of the table ; it is, in fact, not required at all except to avoid the danger of the point splintering. The girdle should be as thin as is compatible with strength sufficient to prevent chipping in the process of mounting the stone ; if it were left thick, the rough edge would be visible by reflection at the lower facets, and would, especially if at all dirty, seriously affect the quality of the stone. The shape of the stone is largely determined by the sizes of the templets in the crown and the pavilions in the base as compared with that of the table, or, what comes to the same thing, by the inclinations at which they are cut to that facet. If the table had actually half the width of the stone, the
FASHIONING OF GEM-STONES 95
angle l between it and a templet would be exactly half a right angle or 45°; it is, however, made somewhat smaller, namely, about 40°. A pavilion, being parallel to a templet, makes a similar angle with the culet. The cross facets are more steeply inclined, and make an angle of about 45° with the table or the culet, as the case may be. The star facets, on the other hand, slant per- ceptibly less, and make an angle of only about 26° with the table. A latitude of some 4° or 5° is possible without seriously affecting the ' fire ' of the stone.
The object of the disposition of the facets on a brilliant is to assure that all the light that enters the stone, principally by way of the table, is wholly reflected from the base and emerges through the crown, preferably by way of the inclined facets. A brilliant-cut diamond, if viewed with the table between the observer and the light, appears quite dark except for the small amount of light escaping through the culet. Light should therefore fall on the lower facets at angles greater than the critical angle of total- reflection, which for diamond is 24° 26'. The pavilions should be inclined properly at double this angle, or 48° 52', to the culet; but a ray that emerges at a pavilion in the actual arrangement entered the table at nearly grazing incidence, and the amount of light entering this facet at such acute perspective is negligible. On the other hand, after reflection at the base light must, in order to emerge, fall on the crown at less than the critical angle
1 In accordance with the usual custom the angle between the facets is taken as that between their normals, or the supplement of the salient angle.
96
GEM-STONES
of total-reflection. In Fig. 50 are shown diagram matically the paths of rays that entered the table in divers ways. The ray emerging again at the table suffers little or no dispersion and is almost white, but those coming out through the inclined facets are split up into the rainbow effect, known as 'fire,' for which diamond is so famous. It is in order that so much of the light entering by the
FlG. 50. — Course of the Rays of Light passing through a Brilliant.
table may emerge through the inclined facets of the crown that the pavilions are inclined at not much more than 40° to the culet. It might be suggested that instead of being faceted the stone should be conically shaped, truncated above and nearly complete below. The result would no doubt be steadier, but, on the other hand, far less pleasing It is the ever-changing nuance that chiefly attracts the eye ; now a brilliant flash of purest white, anon
FASHIONING OP GEM-STONES 9?
a gleam of cerulean blue, waxing to richest orange and dying in a crimson glow, all intermingled with the manifold glitter from the surface of the stone. Absolute cleanliness is essential if the full beauty of any stone is to be realized, but this is particularly true of diamond. If the back of the stone be clogged with grease and dirt, as so often happens in claw-set rings, light is no longer wholly reflected from the base ; much of it escapes, and the amount of ' fire ' is seriously diminished.
Needless to state, lapidaries make no careful angular measurements when cutting stones, but judge of the position of the facets entirely by eye. It sometimes therefore happens that the permissible limits are overstepped, in which event the stone is dead and may resist all efforts to vivify it short of the heroic course of re-cutting it, too expensive a treatment in the case of small stones.
The factors that govern the properties of a brilliant-cut stone are large colour-dispersion, high refraction, and freedom from any trace of intrinsic colour. The only gem-stone that can vie with diamond in these respects is zircon. Although it is rare to find a zircon naturally without colour, yet many kinds are easily deprived of their tint by the application of heat. A brilliant-cut zircon is, indeed, far from readily distinguished by eye from diamond, and has probably often passed as one, but it may easily be identified by its large double refraction (cf. p. 41) and inferior hardness. The remaining colourless stones, such as white sapphire, topaz, and quartz (rock-crystal), have insufficient refractivity to give total-reflection at the base, and, moreover, they are comparatively deficient in ' fire.' 7
98 GEM-STONES
A popular style of cutting which is much in vogue for coloured stones is the step- or trap-cut, consisting of a table and a series of facets with parallel horizontal edges (Figs. 51—52) above and below the girdle ; in recent jewellery, however, the top of the stone is often brilliant-cut The contour may be oblong, square, lozenge, or heart-shaped, or have less regular forms. The table is sometimes slightly rounded. Since the object of this style is primarily to display the intrinsic colour of
view). brilliant play of light from the
interior, no attempt is made to secure total-reflection at the lower facets. The stone therefore varies in depth according to its tint ; if dark, it is cut shallow, lest light be wholly absorbed within, and the stone appear practically opaque, but if light, it is cut deep, in order to secure fullness of tint. Much precision in shape and disposition of the facets is not demanded, and the stones are usually cut in such a way that, provided the desired effect is ob- FlG;, S2.-SteP- or
r . Trap -Cut (side
tamed, the weight is kept as great view). as possible; we may recall that stones are sold by weight In considering what will be the optical effect of any particular shape, regard must be had to the effective colour of the transmitted light. For instance, although sapphire and ruby belong to the same species and have the same refractive indices, yet, since the former transmits mainly blue and the latter red light, they have for practical purposes appreciably different
FASHIONING OF G&M-STONES 99
indices, and lapidaries find it therefore possible to cut the base of ruby thicker than that of sapphire, and thus keep the weight greater. It is instructive too what can be done with the most unpromising material by the exercise of a little ingenuity. Thus Ceylon sapphires are often so irregularly coloured that considerable skill is called for in cutting them. A stone may, for instance, be almost colourless except for a single spot of blue ; yet, if the stone be cut steeply and the spot be brought to the base, the effect will be precisely the same as if the stone were uniformly coloured, because all the light emerging from the stone has passed through the spot at the base and therefore been tinted blue.
The mechanism employed in the fashioning of gem-stones is simple in character, and comprises merely metal plates or wheels for slitting, and discs or laps for grinding and polishing the stones, the former being set vertically and rotated about horizontal spindles, and the latter set horizontally and rotated about vertical spindles. Mechanical power is occasionally used for driving both kinds of apparatus, but generally, especially in slitting and in delicate work, hand-power is preferred. In the East native lapidaries make use of vertical wheels (Plate XIII) also for grinding and polishing stones, which explains why native-cut stones never have truly plane facets ; it will be noticed from the picture that a long bow is used to drive the spindle.
Owing to the unique hardness of diamond it can be fashioned only by the aid of its own powder. The process differs therefore materially from the
ioo GEM-STONES
cutting of the remaining gem-stones, and will be described separately. Indeed, so different are the two classes of work that firms seldom habitually undertake both.
The discovery of the excellent cleavage of diamond enormously reduced the labour of cutting large stones. A stone containing a bad flaw may be split to convenient shape in as many minutes as the days or even weeks required to grind it down. The improvement in the appliances and the provision of ample mechanical power has further accelerated the process and reduced the cost. Two years were occupied in cutting the diamond known as the Pitt or Regent, whereas in only six months the colossal Cullinan was shaped into two large and over a hundred smaller stones with far less loss of material.
Although the brilliant form was derived from the regular octahedron, it by no means follows that, because diamond can be cleaved to the latter form, such is the initial step in fashioning the rough mass. The aim of the lapidary is to cut the largest possible stone from the given piece of rough, and the finished brilliant usually bears no relation whatever to the natural octahedron. The cleavage is utilized only to free the rough of an awkward and useless excres- cence, or of flaws. Although the octahedron is one of the common forms in which diamond is found, it is rarely regular, and oftener than not one of the larger faces is made the table.
The old method, which is still in use, for roughly fashioning diamonds is that known as bruting, from the French word, bmtage, for the process, or as shaping. Two stones of about the same size are selected, and are firmly attached by means of a hard
FASHIONING OF GEM-STONES 101
cement to the ends of two holders, which are held one in each hand, and rubbed hard, one against the other, until surfaces of the requisite size are developed on each stone. During the process the stones are held over a small box, which catches the precious powder. A fine sieve at the bottom of the box allows the powder to fall through into a tray underneath, but holds back anything larger. By means of two vertical pins placed one on each side of the box the holders are retained more easily in the desired position, and the work is thrown mainly on the thumbs. This work continued day after day has a very disfiguring effect upon the hands despite the thick gloves that are worn to protect them ; the skin of the thumbs grows hard and horny, and the first and second fingers become swollen and distorted. When the surfaces have thus been formed, the stone is handed to the polisher, who works them into the correct shape and afterwards polishes them, the stone passing backwards and forwards several times between the cutter and the polisher. The table, four templets, culet and four pavilions are first formed and polished, so that the table has a square shape. Next the quoins are developed and polished, and finally the small facets are polished on, not being shaped first. In modern practice the process of bruting has been modified in some cases by the introduction of machinery, and the facets are ground on, with considerable improvement in the regularity of their size and disposition, and reduction in the amount of polishing required. Moreover, to obviate the loss of material resulting from continued grinding, large stones are first sliced by means of rapidly-re- volving copper wheels charged with diamond powder.
102 GEM-STONES
The laps used for polishing diamonds are made of a particular kind of soft iron, which is found to surpass any other metal in retaining the diamond powder. They are rotated at a high rate of speed, which is about 2000 to 2500 revolutions a minute, and the heat developed by the friction at this speed is too great for a cement to be used ; a solder or fusible alloy, composed of one part tin to three parts lead, therefore takes its place. The solder is held in a hollow cup of brass which is from its shape called a ' dop,' an old Dutch word meaning shell. Its external diameter is ordinarily about i| in. (4 cm.), but larger dops are, of course, used for large stones. A stout copper stalk is attached to the bottom of the dop ; it is visible in the view of the dop shown at e on Plate VI, and two slabs of solder are seen lying in front of the dop. The dop containing the solder is placed in the midst of a non-luminous flame and heated until the solder softens, when it is removed by means of the small tongs, c, and placed upright on a stand such as that shown at a. The long tongs, d, are used for shaping the solder into a cone at the apex of which the diamond is placed. The solder is worked well over the stone so that only the part to undergo polishing is exposed. A diamond in position is shown at/. The top of the stand is saucer-shaped to catch the stone should it accidentally fall off the dop, and to prevent pieces of solder falling on the hand. While still hot, the dop with the diamond in position on the solder is plunged into cold water in order to cool it. The fact that the stone withstands this drastic treatment is eloquent testimony to its good thermal conductivity ; other gem-stones would
POLISHING DIAMONDS
FASHIONING OF GEM-STONES 103
promptly split into fragments. It may be remarked that so high is the temperature at which diamond burns that it may be placed in the gas flame without any fear of untoward results. The dop is now ready for attachment to an arm such as that shown at b ; the stalk of the dop is placed in a groove running across the split end of the arm, and is gripped tight by means of a screw worked by the nut which is visible in the picture.
Four such arms, each with a dop, are used with the polishing lap (Plate VII), and each stands on two square legs on the bench. Pins, /, in pairs are fixed to the bench to prevent the arms being carried round by the friction ; one near the lap holds the arm not far from the dop, and the other engages in a strong metal tongue, which is best seen at the end of the arm b on Plate VI. Though the arm, which is made of iron, is heavy, yet for polishing purposes it is insufficient, and additional lead weights are laid on the top of it, as in the case of the arm at the back on Plate VII. The copper stalk is strong, yet flexible, and can be bent to suit the position of the facet to be polished; on Plate VII the dops a and b are upright, but the other two are inclined. In addition to the powder resulting from bruting, boart, i.e. diamonds useless for cutting, are crushed up to supply polishing material, and a little olive oil is used as a lubricant. Owing to the friction so much heat is developed that even the solder would soften after a time, and therefore, as a precaution, the dop is from time to time cooled by immersion in water. The stone has constantly to be re-set, about six being the maximum even of the tiny facets near the girdle that can be dealt with by
1 04 GEM-STONES
varying the inclination of the dop. As the work approaches completion the stone is frequently in- spected, lest the polishing be carried too far for the development of the proper amount of ' fire.' When finished, the stones are boiled in sulphuric acid to remove all traces of oil and dirt.
The whole operation is evidently rough and ready in the extreme ; but such amazing skill do the lapidaries acquire, that even the most careful in- spection by eye alone would scarce detect any want of proper symmetry in a well-cut stone.
The fashioning of coloured stones, as all the gem-stones apart from diamond are termed in the jewellery trade, is on account of their inferior hardness a far less tedious operation. They are easily slit, for which purpose a vertical wheel (Plate VIII) made of soft iron is used ; it is charged with diamond dust and lubricated with oil, generally, paraffin. When slit to the desired size, the stone is attached to a conveniently shaped holder by means of a cement, the consistency of which varies with the hardness of the stone. It is set in the cement in such a way that the plane desired for the table facet is at right angles to the length of the holder, and the whole of the upper part or crown is finished before the stone, is removed from the cement. The lower half or base is treated in a similar manner. Thus in the process of grinding and polishing the stone is only once re-set ; as was stated above, diamond demands very different treatment. Again, all coloured stones are ground down without any intermediate operation corresponding to bruting. The holder is merely held in the hand, but to maintain its position more exactly its other end,
PLATE VIII
ING COLOURED STi'NKS
VCETING MACHI
FASHIONING OF GEM-STONES 105
which is pointed, is inserted in one of the holes that are pierced at intervals in a vertical spindle placed at a convenient distance from the lap (Plate VIII), which one depending upon the inclination of the facet to be formed. For hard stones, such as ruby and sapphire, diamond powder is generally used as the abrasive agent, while for the softer stones emery, the impure corundum, is selected ; in recent years the artificially prepared carborundum, silicide of carbon corresponding to the formula CSi, which is harder than corundum, has come into vogue for grinding purposes, but it is unfortunately useless for slitting, because it refuses to cling to the wheel. To efface the scratches left by the abrasive agent and to impart a brilliant polish to the facets, material of less hardness, such as putty-powder, pumice, or rouge, is employed ; in all cases the lubricant is water. The grinding laps are made of copper, gun-metal, or lead ; and pewter or wooden laps, the latter sometimes faced with cloth or leather, are used for polishing. As a general rule, the harder the stone the greater the speed of the lap.
As in the case of diamond, the lapidary judges of the position of the facet entirely by eye and touch, but a skilled workman can develop a facet very close to the theoretical position. During recent years various devices have been invented to enable him to do his work with greater facility. A machine of this kind is illustrated on Plate IX. The stone is attached by means of cement to the blunt end, d, of the holder, b, which is of the customary kind, while the other end is inserted in a hole in a wooden piece, a} which is adjustable in height by means of
io6 GEM-STONES
the screw above it. The azimuthal positions of the facets are arranged by means of the octagonal collar, c, the sides of which are held successively in turn against the guide, e. The stand itself is clamped to the bench. The machine is, however, little used except for cheap stones, because it is too accurate and leads to waste of material. Stones are sold by weight, and so long as the eye is satisfied, no attempt is made to attain to absolute symmetry of shape.
The pictures on Plates X-XIII illustrate lapidaries' workshops in various parts of the world. The first two show an office and a workshop situated in Hatton Garden, London ; in the former certain of the staff are selecting from the parcels stones suit- able for cutting. The third depicts a more primitive establishment at Ekaterinburg in the Urals. The fourth shows a typical French family — pere, mere, et fits — in the Jura district, all busily engaged ; on the table will be noticed a faceting machine of the kind described above. In the fifth picture a native lapidary in Calcutta is seen at work with the driving bow in his right, and the stone in his left, hand.
A curious difference exists in the systems of charging for cutting diamonds and coloured stones. The cost of cutting the latter is reckoned by the weight of the finished stone, the rate varying from is. to 8s. a carat according to the character of the stone and the difficulty of the work ; while in the case of diamonds, on the other hand, the weight of the rough material determines the cost, the rate being about los. to 403. a carat according to the size, which on the average is equivalent to about 303. to 1 2 os. a carat calculated on the weight
FASHIONING OF GEM-STONES 107
of the finished stone. The reason of the distinction is obviously because the proper proportions in a brilliant-cut diamond must be maintained, whatever be the loss in weight involved ; in coloured stones the shape is not of such primary importance.
When finished, the stone finds its way with others akin to it to the manufacturing jeweller's establishment, where it is handed to the setter, who mounts it in a ring, necklace, brooch, or whatever article of jewellery it is intended for. The metal used in the groundwork of the setting is generally gold, but platinum is also employed where an unobtrusive and untarnishable metal is demanded, and silver finds a place in cheaper jewellery, although it is seriously handicapped by its susceptibility to the blackening influence of the sulphurous fumes present in the smoke-laden atmosphere of towns. The stone may be either embedded in the metal or held by claws. The former is by far the safer, but the latter the more elegant, and it has the advantage of exposing the stone d jour, to use the French jewellers' expression, so that its genuineness is more evidently testified. It is very important that the claw setting be periodically examined, lest the owner one day experience the mortification of finding that a valuable stone has dropped out ; gold, owing to its softness, wears away in course of time.
Up to quite recent years modern jewellery was justly open to the criticism that it was lacking in variety, that little attempt was made to secure harmonious association in either the colour or the lustre of the gem-stones, and that the glitter of the gold mount was frequently far too obtrusive. Gold
1 08 GEM-STONES
consorts admirably with the rich glow of ruby, but is quite unsuited to the gleaming fire of a brilliant. Where the metal is present merely for the mechanical purpose of holding the stones in position, it should be made as little noticeable as possible. The artistic treatment of jewellery is, however, receiving now adequate attention in the best Paris and London houses. Some recent designs are illustrated on Plates IV and V.
PLA TE XIII
CHAPTER XIII NOMENCLATURE OF PRECIOUS STONES
THE names in popular use for the principal gem-stones may be traced back to very early times, and, since they were applied long before the determinative study of minerals had become a science, their significance has varied at different dates, and is even now far from precise. No ambiguity or confusion could arise if jewellers made use of the scientific names for the species, but most of them are unknown or at least unfamiliar to those unversed in mineralogy, and to banish old-established names is undesirable, even if the task were not hopeless. The name selected for a gem-stone may have a very important bearing on its fortunes. When the love-sick Juliet queried ' What's in a name ? ' her mind was wandering far from jewels ; for them a name is everything. The beautiful red stones that accompany the diamond in South Africa were almost a drug in the market under their proper title — garnet, but command a ready sale under the misnomer ' Cape-ruby.' To many minds there is a subtle satisfaction in the possession of a stone which is assumed to be a sort of ruby that would be destroyed by the know- ledge that the stone really belonged to the Cinderella species of gem-stones — the despised garnet. For
no GEM-STONES
similar reasons it was deemed advisable to offer the lustrous green garnet found some thirty and odd years ago in the Ural Mountains as ' olivine/ not a happy choice since their colour is grass- rather than olive-green, apart from the fact that the term is in general use in science for the species known in jewellery as peridot.
The names employed in jewellery are largely based upon the colour, the least reliable from a determinative point of view of all the physical characters of gem-stones. Qualifying terms are employed to distinguish stones of obviously different hardness. ' Oriental ' distinguishes varieties of corundum, but does not imply that they necessarily came from the East ; the finest gem-stones originally reached Europe by that road, and the hardest coloured stones consequently received that term of distinction.
Nearly all red stones are grouped under the name ruby, which is derived from a Latin word, ruber, meaning red, or under other names adapted from it, such as rubellite, rubicelle. It is properly applied to red corundum ; ' balas ' ruby is spinel, which is associated with the true ruby at the Burma mines and is similar in appearance to it when cut, and ' Cape ' ruby, is, as has been stated above, a garnet from South Africa. Rubellite is the lovely rose-pink tourmaline, fine examples of which have recently been discovered in California, and rubicelle is a less pronouncedly red spinel. Sapphire is by far the oldest and one of the most interesting of the words used in the language of jewels. It occurs in Hebrew and Persian, ancient tongues, and means blue. It was apparently employed for lapis lazuli
NOMENCLATURE OF PRECIOUS STONES 1 1 1
or similar substance, but was transferred to the blue corundum upon the discovery of this splendid stone. Oblivious of the real meaning of the word, jewellers apply it in a quasi-generic sense to all the varieties of corundum with the exception of the red ruby, and give vent to such incongruous expressions as ' white sapphire,' ' yellow sapphire ' ; it is true such stones often contain traces of blue colour, but that is not the reason of the terms. ' Brazilian ' sapphire is blue tourmaline, a somewhat rare tint for this species. The curious history of the word topaz will be found below in the chapter dealing with the species of that name. It has always denoted a yellow stone, and at the present day is applied by jewellers indis- criminately to the true topaz and citrine, the yellow quartz, the former, however, being sometimes dis- tinguished by the prefix ' Brazilian.' ' Oriental ' topaz is corundum, and 'occidental' topaz is a term occasionally employed for the yellow quartz. Emerald, which means green, was first used for chrysocolla, an opaque greenish stone (p. 288), but was afterwards applied to the priceless green variety of beryl, for which it is still retained. ' Oriental ' emerald is corundum, c Brazilian ' emerald in the eighteenth century was a common term for the green tourmaline recently introduced to Europe, and ' Uralian ' emerald has been tentatively suggested for the green garnet more usually known as 'olivine.' Amethyst is properly the violet quartz, but with the prefix ' oriental ' it is also applied to violet corundum, though some jewellers, use it for the brilliant quartz, with purple and white sectors, from Siberia. Almandine, which is derived from the name of an Eastern mart for precious stones, has
ii2 GEM-STONES
come to signify a stone of columbine-red hue, principally garnet, but with suitable qualification corundum and spinel also.
The nomenclature of jewellery tends to suggest relations between the gem-stones for which there is no real foundation, and to obscure the essential identity, except from the point of view of colour, of sapphire and ruby, emerald and aquamarine, cairngorm and amethyst.
CHAPTER XIV MANUFACTURED STONES r
THE initial step in the examination of a crystallized substance is to determine its physical characters and to resolve it by chemical analysis into its component elements ; the final, and by far the hardest, step is to build it up or synthetic- ally prepare it from its constituents. Unknown to the world at large, work of the latter kind has long been going on within the walls of laboratories, and as the advance in knowledge placed in the hands of experimenters weapons more and more compar- able with those wielded by nature, their efforts have been increasingly successful. So stupendous, how- ever, are the powers of nature that the possibility of reproducing, by human agency, the treasured stones which are extracted from the earth in various parts of the globe at the cost of infinite toil and labour has always been derided by those ignorant of what had already been accomplished. Great, therefore, was the consternation and the turmoil when concrete evidence that could not be gainsaid showed that man's restless efforts to bridle nature to his will were not in vain, and congresses of all the high-priests of jewellery were hastily con- vened to ban such unrighteous products, with what ultimate success remains to be seen. 8 "3
114 GEM-STONES
Crystallization may be caused in four different ways, of which the second alone has as yet yielded stones large enough to be cut —
1. By the separation of the substance from a saturated solution. In nature the solvent may not be merely hot water, or water charged with an acid, but molten rock, and the temperature and the pressure may be excessively high.
2. By the solidification of the liquefied substance upon cooling. Ice is a familiar example of this type.
3. By the sublimation of the vapour of the sub- stance, which means the direct passage from the vapour to the solid state without traversing the usually intervening liquid state. It is usually the most difficult of attainment of the four methods ; the most familiar instance is snow.
4. By the precipitation of the substance from a solution when set free by chemical action.
Other things being equal, the simpler the com- position the greater is the ease with which a sub- stance may be expected to be formed ; for, instead of one complex substance, two or more different substances may evolve, unless the conditions are nicely arranged. Attempts, for instance, to produce beryl might result instead in a mixture of chryso- beryl, phenakite, and quartz.
By far the simplest in composition of all the precious stones is diamond, which is pure crystallized carbon ; but its manufacture is attended by well- nigh insuperable difficulties. If carbon be heated in air, it burns at a temperature well below its melting point ; moreover, unless an enormously high pressure is simultaneously applied, the product
MANUFACTURED STONES 1 1 5
is the other form of crystallized carbon, namely, the comparatively worthless graphite. Moissan's in- teresting course of experiments were in some degree successful, but the tiny diamonds were worthless as jewels, and the expense involved in their manu- facture was out of all proportion to any possible commercial value they might have.
Next to diamond the simplest substances among precious stones are quartz (crystallized silica) and corundum (crystallized alumina). The crystallization of silica has been effected in several ways, but the value in jewellery of quartz, even of the violet variety, amethyst, is not such as to warrant its manufacture on a commercial scale. Corundum, on the other hand, is held in high esteem ; rubies and sapphires, of good colour and free from flaws, have always commanded good prices. The question of their production by artificial means has therefore more than academic interest.
Ever since the year 1837, when Gaudin produced a few tiny flakes, French experimenters have steadily prosecuted their researches in the crystallization of corundum. Frdmy and Feil, in 1877, were the first to meet with much success. A portion of one of their crucibles lined with glistening ruby flakes is exhibited in the British Museum (Natural History).
In 1885 the jewellery market was completely taken by surprise by the appearance of red stones, emanating, so it is alleged, from Geneva ; having the physical characters of genuine rubies, they were accepted as, and commanded the prices of, the natural stones. It was eventually discovered that they had resulted from the fusion of a number of
n6
GEM-STONES
fragments of natural rubies in the oxy-hydrogen flame. The original colour was driven off at that high temperature, but was revived by the previous addition of a little bichromate of potassium. Owing to the inequalities of growth, the cracks due to rapid cooling, the inclusion of air-bubbles, often so numerous as to cause a cloudy appear- ance, and, above all, the un- natural colour, these recon- structed stones, as they are termed, were far from satisfac- tory, but yet they marked such an advance on anything that had been accomplished before that for some time no suspicion was aroused as to their being other than natural stones.
A notable advance in the synthesis of corundum, par- ticularly of ruby, was made in 1 904, when Verneuil, who had served his apprenticeship to science under the guidance of Fremy, invented his ingenious inverted form of blowpipe (Fig- 53)> which enabled him to overcome the difficulties that had baffled earlier investigators, and to manufacture rubies vying in appearance after cutting with the best of nature's productions. The blowpipe consisted of two tubes, of which the upper, E, wide above, was constricted below, and passing down the centre of the lower, F, terminated just above the orifice
FIG. 53. — Verneuil's In- verted Blowpipe.
MANUFACTURED STONES 117
of the latter in a fine nozzle. Oxygen was admitted at C through the plate covering the upper end of the tube, E. A rod, which passed through a rubber collar in the same plate, supported inside the tube, E, a vessel, D, and at the upper end terminated in a small plate, on which was fixed a disc, B. The hammer, A, when lifted by the action of an electro- magnet and released, fell by gravity and struck the disc. The latter could be turned about a horizontal axis placed eccentrically, so that the height through which the hammer fell and the consequent force of the blow could be regulated. The rubber collar, which was perfectly gas-tight, held the rod securely, but allowed the shocks to be transmitted to the vessel, D, an arrangement of guides maintaining the slight motion of the vessel strictly vertical. This vessel, which carried the alumina powder used in the manufacture of the stone, had as its base a cylindrical sieve of fine mesh. The succession of rapid taps of the hammer caused a regular feed of powder down the tube, the amount being regulated by varying the height through which the hammer fell. Hydrogen or coal-gas was admitted at G into the outer tube, F, and in the usual way met the oxygen just above the orifice, L. To exclude irregular draughts, the flame was surrounded by a screen, M, which was provided with a mica window, and a water-jacket, K, protected the upper part of the apparatus from excessive heating.
The alumina was precipitated from a solution of pure ammonia - alum, (NH4)2SO4.A12(SO4)3.24H2O, in distilled water by the addition of pure ammonia, sufficient chrome-alum also being dissolved with the ammonia-L^um to furnish about 2\ per cent.
1 1 8 GEM-STONES
of chromic oxide in the resulting stone. The powder, carefully prepared and purified, was placed, as has been stated above, in the vessel, D, and on reaching the flame at the orifice it melted, and fell as a liquid drop, N, upon the pedestal, P, which was formed of previously fused alumina. This pedestal was attached by a platinum sleeve to an iron rod, Q, which was provided with the necessary screw adjustments, R and S, for centring and lowering it as the drop grew in size. Great care was exercised to free the powder from any trace of potassium, which, if present, imparted a brownish tinge to the stone. The pressure of the oxygen, low initially both to prevent the pedestal from melting, and to keep the area of the drop in contact with the pedestal as small as possible, FIG. 54.— 'Boule,' because otherwise flaws tended to D Car S aped start on cooling, was gradually in-» creased until the flame reached the critical temperature which kept the top of the drop melted, but not boiling. The supply of powder was at the same time carefully proportioned to the pressure. The pedestal, P, was from time to time lowered, and the drop grew in the shape of a pear (Fig. 54), the apex of which was downwards and adhered to the pedestal by a narrow stalk. As soon as the drop reached the maximum size possible with the size of the flame, the gases were sharply and simultaneously cut off. After ten minutes or so the drop was lowered from the chamber, M, by the screw, S, and when quite cold was removed from the pedestal.
Very few changes have been made in the method when adapted to commercial use. Coal-gas has,
BI.OWI'II'E USED FOR THE MANUFACTURE OF RUBIES AND SAPPHIRES
MANUFACTURED STONES 119
however, entirely replaced the costly hydrogen, and the hammer is operated by a cam instead of an electromagnet, while, as may be seen from the view of a gem-stone factory (Plate XIV), a number of blow- pipes are placed in line so that their cams are worked by the same shaft, a. The fire-clay screen, b, surrounding the flame is for convenience of re- moval divided into halves longitudinally, and a small hole is left in front for viewing the stone during growth, a red glass screen, c, being provided in front to protect the eyes from the intense glare. Half the fire-clay screen of the blowpipe in the centre of the Plate has been removed to show the arrangement of the interior. The centring and the raising and lowering apparatus, d, have been modified. The process is so simple that one man can attend to a dozen or so of these machines, and it takes only one hour to grow a drop large enough to be cut into a ten-carat stone.
The drops, unless the finished stone is required to have a similar pear shape, are divided longitudin- ally through the central core into halves, which in both shape and orientation are admirably suited to the purposes of cutting ; as a general rule, the drop splits during cooling into the desired direction of its own accord.
Each drop is a single crystalline individual, and not, as might have been anticipated, an alumina glass or an irregular aggregation of crystalline fragments, and, if the drop has cooled properly, the crystallographic axis is parallel to the core of the pear. The cut stone will therefore have not only the density and hardness, but also all the optical characters — refractivity, double refraction,
120 GEM-STONES
dichroism, etc. — pertaining to the natural species, and will obey precisely the same tests with the re- fractometer and the dichroscope. Were it not for certain imperfections it would be impossible to distinguish between the stones formed in Nature's vast workshop and those produced within the confines of a laboratory. The artificial stones, however, are rarely, if ever, free from minute air-bubbles (Fig. 55), which can FIG. 55.— Bubbles easily be seen with an ordinary and Curved Strise lens Their spherical shape differ- in Manufactured . , .. , , Ruby. entiates them from the plane- sided cavities not infrequently visible in a natural stone (Fig. 56). Moreover, the colouring matter varies slightly, but imper- ceptibly, in successive shells, and consequently in the finished stone a careful eye can discern the curved striations (Fig. 55) corresponding in shape to the original shell. In a natural stone, on the other hand, although zones of different colours or varying shades are not uncommon, the resulting striations are straight (Fig. 56), corresponding to the ^ plane faces of the original crystal in Naturai Ruby. form. By sacrificing material it might be possible to cut a small stone free from bubbles, but the curved striations would always be present to betray its origin.
The success that attended the manufacture of ruby encouraged efforts to impart other tints to crystallized alumina. By reducing the percentage amount of chromic oxide, pink stones were turned
MANUFACTURED STONES 121
out, in colour not unlike those Brazilian topazes, the original hue of which has been altered by the appli- cation of heat. These artificial stones have there- fore been called ' scientific topaz ' ; of course, quite wrongly, since topaz, which is properly a fluo-silicate of aluminium, is quite a different substance.
Early attempts made to obtain the exquisite blue tint of the true sapphire were frustrated by an un- expected difficulty. The colouring matter, cobalt oxide, was not diffused evenly through the drop, but was huddled together in splotches, and it was found necessary to add a considerable amount of magnesia as a flux before a uniform distribution of colour could be secured. It was then discovered that, despite the colour, the stones had the physical characters, not of sapphire, but of the species closely allied to it, namely, spinel, aluminate of magnesium. By an unsurpassable effort of nomenclature these blue stones were given the extraordinary name of ' Hope sapphire,' from fanciful analogy with the famous blue diamond which was once the pride of the Hope collection. A blue spinel is occasionally found in nature, but the actual tint is somewhat different. These manufactured stones have the disadvantage of turning purple in artificial light. By substituting lime for magnesia as a flux, Paris, a pupil of Verneuil's, produced blue stones which were not affected to the same extent. The difficulty was at length overcome at the close of 1909, when Verneuil, by employing as tinctorial agents 0*5 per cent, of titanium oxide and 1-5 per cent, of magnetic iron oxide, succeeded in producing blue corundum ; it, however, had not quite the tint of sapphire. Stones subsequently manufactured, which were
122 GEM-STONES
better in colour, contained about 0-12 per cent of titanium oxide, but no iron at all.
By the addition to the alumina of a little nickel oxide and vanadium oxide respectively, yellow and yellowish green corundums have been obtained. The latter have in artificial light a distinctly reddish hue, and have therefore been termed 'scientific alexandrite'; of course, quite incorrectly, since the true alexandrite is a variety of chrysoberyl, alumin- ate of beryllium, a very different substance.
If no colouring matter at all be added and the alum be free from potash, colourless stones or white sapphires are formed, which pass under the name ' scientific brilliant.' It is scarcely necessary to remark that they are quite distinct from the true brilliant, diamond.
The high prices commanded by emeralds, and the comparative success that attended the recon- struction of ruby from fragments of natural stones, suggested that equal success might follow from a similar process with powdered beryl, chromic oxide being used as the colouring agent. The resulting stones are, indeed, a fair imitation, being even pro- vided with flaws, but they are a beryl glass with lower specific gravity and refractivity than the true beryl, and are wrongly termed ' scientific emerald.' Moreover, recently most of the stones so named on the market are merely green paste.
It is unfortunate that the real success which has been achieved in the manufacture of ruby and sap- phire should be obscured by the ill-founded claims tacitly asserted in other cases.
At the time the manufactured ruby was a novelty it fetched as much as £,6 a carat, but as soon as
MANUFACTURED STONES 123
it was discovered that it could easily be differenti- ated from the natural stone, a collapse took place, and the price fell abruptly to 305., and eventually to 5s. and even is. a carat. The sapphires run slightly higher, from 2s. to ?s. a carat. The prices of the natural stones, which at first had fallen, have now risen to almost their former level. The extreme disparity at present obtaining between the prices of the artificial and the natural ruby renders the fraudulent substitution of the one for the other a great temptation, and it behoves purchasers to beware where and from whom they buy, and to be suspicious of apparently remarkable bargains, especi- ally at places like Colombo and Singapore where tourists abound. It is no secret that some thousands of carats of manufactured rubies are shipped annu- ally to the East. Caveat emptor.
CHAPTER XV IMITATION STONES
THE beryl glass mentioned in the previous chapter marks the transition stage between manufactured stones which in all essential characters are identical with those found in nature, and arti- ficial stones which resemble the corresponding natural stone in outward appearance only. In a sense both sorts may be styled artificial, but it would be mis- leading to confound them under the same appel- lation.
Common paste,1 which is met with in drapery goods and cheap ornaments in general — hat-pins, buckles, and so forth — is composed of ordinary crown-glass or flint-glass, the refractive indices being about 1*53 and i'63 respectively. The finest quality, which is used for imitations of brilliants, is called ' strass.' It is a dense lead flint- glass of high refraction and strong colour - dis- persion, consisting of 38*2 per cent, of silica, 53^3 red lead (oxide of lead), and 7'8 potassium carbon- ate, with small quantities of soda, alumina, and other substances. How admirable these imitations may be, a study of the windows of a shop devoted
1 The word paste is derived from the Italian, pasta, food, being suggested by the soft plastic nature of the material used to imitate gems.
IMITATION STONES 125
to such things will show. Unfortunately the addition of lead, which is necessary for imparting the requisite refraction and ' fire ' to the strass, renders the stones exceedingly soft. All glass yields to the file, but strass stones are scratched even by ordinary window-glass. If worn in such a way that they are rubbed, they speedily lose the brilliance of their polish, and, moreover, they are susceptible to attack by the sulphurous fumes present in the smoky air of towns, and turn after a time a dirty brown in hue. When coloured stones are to be imitated, small quantities of a suitable metallic oxide are fused with the glass ; cobalt gives rise to a royal-blue tint, chromium a ruby red, and manganese a violet. Common paste is not highly refractive enough to give satisfactory results when cut as a brilliant, and the bases are therefore often coated with quicksilver, or, in the case of old jewellery, covered with foil in the setting, in order to secure more complete reflection from the interior. The fashioning of these imitation stones is easy and cheap. Being moulded, they do not require cutting, and the polishing of the facets thus formed is soon done on account of the softness of the stones.
A test with a file readily differentiates paste stones from the natural stones they pretend to be. Being necessarily singly refractive, they are, of course, lacking in dichroism, and their refractivity seldom accords even approximately with that of the corresponding natural stone.
In order to meet the test for hardness the doublet was devised. Such a stone is composed of two parts — the crown consisting of colourless
126 GEM-STONES
quartz or other inexpensive real and hard stone, and the base being made up of coloured glass. When the imitation, say of a sapphire, is intended to be more exact, the crown is made of a real sapphire, but one deficient in colour, the requisite tint being obtained from the paste forming the under part of the doublet. In case the base should also be tested for hardness the triplet has been devised. In this the base is made of a real stone also, and the coloured paste is confined to the girdle section, where it is hidden by the setting. Sapphires and emeralds of indifferent colour are sometimes slit across the girdle; the interior surfaces are polished, and colouring matter is introduced with the cement, generally Canada balsam, which is used to re-unite the two portions of the stone together. All such imitations may be detected by placing the stone in oil, when the surfaces separating the portions of the composite stone will be visible, or the binding cement may be dissolved by immersing the stone, if unmounted, in boiling water, or in alcohol or chloroform, when the stone will fall to pieces.
The glass imitations of pearls, which have be- come very common in recent years, may, apart from their inferior iridescence, be detected by their greater hardness, or by the apparent doubling of, say, a spot of ink placed on the surface, owing to reflection from the inner surface of the glass shell. They are made of small hollow spheres formed by blowing. Next to the glass comes a lining of parchment size, and next the under lining, which is the most important part of the imitation, consisting of a preparation of fish scales called Essence d' Orient. When the lining
IMITATION STONES 127
is dry, the globe is filled with hot wax to impart the necessary solidity. In cheap imitations the glass balls are not lined at all, but merely heated with hydrochloric acid to give an iridescence to the sur- face ; sometimes they are coated with wax, which can be scraped off with a knife.
PART II— SECTION A PRECIOUS STONES
CHAPTER XVI DIAMOND
DIAMOND has held pride of place as chief of precious stones ever since the discovery of the form of cutting known as the ' brilliant ' revealed to full perfection its amazing qualities ; and justly so, since it combines in itself extreme hardness, high refraction, large colour-dispersion, and brilliant lustre. A rough diamond, especially from river gravels, has often a peculiar greasy appearance, and is no more attractive to the eye than a piece of washing-soda. It is therefore easy to understand why the Persians in the thirteenth century placed the pearl, ruby, emerald, and even peridot before it, and writers in the Middle Ages frequently esteemed it below emerald and ruby. The Indian lapidaries, who were the first to realize that diamond could be ground with its own powder, discovered what a wonderful difference the removal of the skin makes in the appearance of a stone. They, however, made no attempt to shape a stone, but merely polished the natural facets, and only added numerous
DIAMOND 129
small facets when they wished to conceal flaws or other imperfections ; indeed, the famous traveller, Tavernier, from whom most of our knowledge of early mining in India is obtained, invariably found that a stone covered with many facets was badly flawed. The full radiant beauty of a diamond comes to light only when it is cut in brilliant form.
Of all precious stones diamond has the simplest composition ; it is merely crystallized carbon, another form of which is the humble and useful graphite, commonly known as ' black-lead.' Surely nature has surpassed all her marvellous efforts in producing from the same element substances with such divergent characters as the hard, brilliant, and transparent diamond and the soft, dull, and opaque graphite. It is, however, impossible to draw any sharp dividing line between the two ; soft diamond passes insensibly into hard graphite, and vice versa. Boart, or bort, as it is sometimes written, is composed of minute crystals of diamond arranged haphazardly ; it possesses no cleavage, its hardness is greater than that of the crystals, and its colour is greyish to blackish. Carbon, carbonado, or black diamond, which is composed of still more minute crystals, is black and opaque, and is perceptibly harder than the crystals. It passes into graphite, which varies in hardness, and may have any density between 2-o and 3-o. Jewellers apply the term boart to crystals or fragments which are of no service as gems ; such pieces are crushed to powder and used for cutting and polishing purposes.
Diamonds, when absolutely limpid and free from flaws, are said to be of the ' first water,' and are most prized when devoid of any tinge of colour 9
1 30 GEM-STONES
except perhaps bluish (Plate I, Fig. i). Stones with a slight tinge of yellow are termed 'off-coloured,' and are far less valuable. Those of a canary-yellow colour (Plate I, Fig. 3), however, belong to a different category, and have a decided attractiveness. Green- ish stones also are common, though it is rare to come across one with a really good shade of that colour. Brown stones, especially in South Africa, are not uncommon. Pink stones are less common, and ruby-red and blue stones are rare. Those of the last-named colour have usually what is known
FIGS. 57-59.— Diamond Crystals.
as a ' steely ' shade, i.e. they are tinged with green ; stones of a sapphire blue are very seldom met with, and such command high prices.
Diamond crystallizes (Figs. 57—59 and Plate I, Fig. 2) in octahedra with brilliant, smooth faces, and occasionally in cubes with rough pitted faces ; sometimes three or six faces take the place of each octahedron face, and the stone is almost spherical in shape. The surfaces of the crystals are often marked with equilateral triangles, which are supposed to represent the effects of incipient combustion. Twinned crystals, in which the two individuals may be connected by a single plane or may be
DIAMOND 131
interpenetrating, a star shape often resulting in the latter case, are common ; sometimes, if of the octahedron type, they are beautifully symmetrical. The rounded crystals are frequently covered with a peculiar gum-like skin which is somewhat less hard than the crystal itself. A large South African stone, weighing 27 grams (130 carats) and octahedral in shape, which was the gift of John Ruskin, and named by him the ' Colenso ' after the first bishop of Natal, is exhibited in the British Museum (Natural History) ; its appearance is, however, marred by its distinctly ' off-coloured ' tint.
The refraction of diamond is single, but local double refraction is common, indicating a state of strain which can often be traced to an included drop of liquid carbonic acid ; so great is the strain that many a fine stone has burst to fragments on being removed from the ground in which it has lain. The refractive index for the yellow light of a sodium flame is 2^4 17 5, and the slight variation from this mean value that has been observed, amounting only to O'OOOl, testifies to the purity of the composition. The colour-dispersion is large, being as much as 0-044, in which respect it surpasses all colourless stones, but is exceeded by sphene and the green garnet from the Urals (cf. p. 217). The lustre of diamond, when polished, is so characteristic as to be termed adamantine, and is due to the combina- tion of high refraction and extreme hardness. Diamond is translucent to the X (Rontgen) rays ; it phosphoresces under the action of radium, and of a high-tension electric current when placed in a vacuum tube, and sometimes even when exposed to strong sunlight. Some diamonds fluoresce in
132 GEM-STONES
sunlight, turning milky, and a few even emit light when rubbed. Crookes found that a diamond buried in radium bromide for a year had acquired a lovely blue tint, which was not affected even by heating to redness. The specific gravity is like- wise constant, being 3*521, with a possible variation from that mean value of 0*005 5 but a greater range, as might be expected, is found in the impure boart. Diamond is by far the hardest substance in nature, being marked I o in Mohs's scale of hardness, but it varies in itself; stones from Borneo and New South Wales are so perceptibly harder than those usually in the lapidaries' hands, that they can be cut only with their own and not ordinary diamond powder, and some difficulty was experienced in cutting them when they first came into the market. It is interesting to note that the metal tantalum, the isolation of which in commercial amount constituted one of the triumphs of chemistry of recent years, has about the same hardness as diamond. Despite its extreme hardness diamond readily cleaves under a heavy blow in planes parallel to the faces of the regular octahedron, a property utilized for shaping the stone previous to cutting it. The fallacious, but not unnatural, idea was prevalent up to quite modern times that a diamond would, even if placed on an anvil, resist a blow from a hammer : who knows how many fine stones have succumbed to this illusory test? The fact that diamond could be split was known to Indian lapidaries at the time of Tavernier's visit, and it would appear from De Boodt that in the sixteenth century the cleavability of diamond was not unknown in Europe, but it was not credited
DIAMOND 133
at the time and was soon forgotten. Early last century Wollaston, a famous chemist and mineral- ogist, rediscovered the property, and, so it is said, used his knowledge to some profit by purchasing large stones, which because of their awkward shape or the presence of flaws in the interior were rejected by the lapidaries, and selling them back again after cleaving them to suitable forms.
It has already been remarked (p. 79) that the interval in hardness between diamond and corundum, which comes next to it in Mohs's scale, is enormously greater than that between corundum and the softest of minerals. Diamond can therefore be cut only with the aid of its own powder, and the cutting of diamond is therefore differentiated from that of other stones, the precious-stone trade being to a large ex- tent divided into two distinct groups, namely, dealers in diamonds, and dealers in all other gem-stones.
The name of the species is derived from the popular form, adiamentem, of the Latin adamantem, itself the alliterative form of the Greek aSa/^a?, meaning the unconquerable, in allusion not merely to the great hardness but also to the mistaken idea already mentioned. Boart probably comes from the Old-French bord or borty bastard.
At the present day diamonds are usually cut as brilliants, though the contour of the girdle may be circular, oval, or drop-shaped to suit the particular purpose for which the stone is required, or to keep the weight as great as possible. Small stones for bordering a large coloured stone may also be cut as roses or points. A perfect brilliant has 5 8 facets, but small stones may have not more than 44, and exceptionally large stones may with advantage have
134 GEM-STONES
many more ; for instance, on the largest stone cut from the Cullinan diamond there are no fewer than 74 facets.
The description of the properties of diamond would not be complete without a reference to the other valuable, if utilitarian, purposes to which it is put Without its aid much of modern engineering work and mining operations would be impossible except at the cost of almost prohibitive expenditure of time and money.
Boring through solid rock has been greatly facilitated by the use of the diamond drill. For this purpose carbonado or black diamond is more serviceable than single crystals, and the price of the former has consequently advanced from a nominal figure up to £3 to £12 a carat. The actual working part of the drill consists of a cast-steel ring. The crown of it has a number of small depressions at regular intervals into which the carbonados are embedded. On revolution of the drill an annular ring is cut, leaving a solid core which can be drawn to the surface. For cooling the drill and for washing away the detritus water is pumped through to the working face. The duration of the carbonados depends on the nature of the rock and the skill of the operator. The most troublesome rock is a sandstone or one with sharp differences in hardness, because the carbonados are liable to be torn out of their setting. An experienced operator can tell by the feel of the drill the nature of the rock at the working face, and by varying the pressure can mitigate the risk of damage to the drill.
The tenacity of diamond renders it most suitable for wire-drawing. The tungsten filaments used in
DIAMOND 135
many of the latest forms of incandescent electric lamps are prepared in this manner.
Diamond powder is used for cutting and turning the hardened steel employed in modern armaments and for other more peaceful purposes.
Although nearly all the gem-stones scratch glass, diamond alone can be satisfactorily employed to cut it along a definite edge. Any flake at random will not be suitable, because it will tear the glass and form a jagged edge. The best results are given by the junction of two edges which do not meet in too obtuse an angle ; two edges of the rhombic dodecahedron meet the requirements admirably. The stones used by the glaziers are minute in size, being not much larger than .a pin's head, and thirty of them on an average go to the carat. They are set in copper or brass. Some little skill is needed to obtain the best results.
The value of a diamond has always been determined largely by the size of the stone, the old rule being that the rate per carat should be multiplied by the square of the weight in carats ; thus, if the rate be £ i o, the cost of a two-carat stone is four times this sum, or £4.0, of a three-carat stone £90, and so on. For a century, from 1750 to 1850, the rate remained almost constant at £4. for rough, £6 for rose-cut, and £8 for brilliant-cut diamonds. Since the latter date, owing to the increase in the supply of gold, the growth of the spending power of the world, and the gradual falling off in the productiveness of the Brazilian fields, the rate steadily increased about 10 per cent, each year, until in 1865 the rate for brilliants was £iB. The rise was checked by the discovery of the South African mines ; moreover,
136 GEM-STONES
since comparatively large stones are plentiful in these mines, the rule of the increase in the price of a stone by the square of its weight no longer holds. The rate for the most perfect stones still remains high, because such are not so common in the South African mines. The classification 1 adopted by the syndicate of London diamond merchants who place upon the market the output of the De Beers group of mines is as follows : — (a) Blue-white, (b} white, (c) silvery Cape, (d) fine Cape, (e) Cape, (/") fine by- water, (g-) by water, (/t) fine light brown, (z) light brown, (/) brown, (/£) dark brown. Bywaters or byes are stones tinged with yellow.
The rate per carat for cut stones in the blue- white and the by water groups is : —
BLUE-WHITE. BYWATER.
5-carat stone . . ^40-60 ^20-25
i „ . 30-40 10-15
\ „ . . 20-25 8-12
J „ . 15-18 6-10
Melee . . . 12-15 5-8
Melee are stones smaller than a quarter of a carat. It will be noticed that the prices depart largely from the old rule ; thus taking the rate for a carat blue-white stone, the price of a five-carat stone should be from £15 0—200 a carat, and for a quarter-carat stone only £7, los. to £10 a carat. There happens to be at the time of writing very little demand for five-carat stones. Of course, the prices given are subject to constant fluctuation depending upon the supply and demand, and the whims of fashion.
1 Cf. below, p. 149.
CHAPTER XVII OCCURRENCE OF DIAMOND
THE whole of the diamonds known in ancient times were obtained from the so-called Golconda mines in India. Golconda itself, now a deserted fortress near Hyderabad, was merely the mart where the diamonds were bought and sold. The diamond-bearing district actually spread over a wide area on the eastern side of the Deccan, ex- tending from the Pinner River in the Madras Presidency northwards to the Rivers Son and Khan, tributaries of the Ganges, in Bundelkhand. The richest mines, where the large historical stones were found, are in the south, mostly near the Kistna River. The diamonds were discovered in sandstone, or conglomerate, or the sands and gravels of river- beds. The mines were visited in the middle of the seventeenth century by the French traveller and jeweller, Tavernier, when travelling on a commission for Louis XIV, and he afterwards published a careful description of them and of the method of working them. The mines seem to have been exhausted in the seventeenth century ; at any rate, the prospecting, which has been spasmodically carried on during the last two centuries, has proved almost abortive. With the exception of the Koh-i-nor, all the large Indian diamonds were probably discovered not long
138 GEM-STONES
before Tavernier's visit. The diamonds known to Pliny, and in his time, were quite small, and it is doubtful if any stones of considerable size came to light before A.D. 1000.
India enjoyed the monopoly of supplying the world's demand for diamonds up to the discovery, in 1725, of the precious stone in Brazil. Small stones were detected by the miners in the gold washings at Tejuco, about eighty miles (129 km.) from Rio de Janeiro, in the Serro do Frio district of the State of Minas Geraes. The discovery naturally caused great excitement. So many diamonds were found that in 1727 something like a slump took place in their value. In order to keep up prices, the Dutch merchants, who mainly controlled the Indian output, asserted that the diamonds had not been found in Brazil at all, but were inferior Indian stones shipped to Brazil from Goa. The tables were neatly turned when diamonds were actually shipped from Brazil to Goa, and exported thence to Europe as Indian stones. This course and the continuous development of the diamond district in Brazil rendered it impossible to hoodwink the world indefinitely. The drop in prices was, however, stayed by the action of the Portuguese government, who exacted such heavy duties and imposed such onerous conditions that finally no one would under- take to work the mines. Accordingly, in 1772 diamond-mining was declared a royal monopoly in Brazil, and such it remained until the severance of Brazil from Portugal in 1834, when private mining was permitted by the new government subject to the payment of reasonable royalties. The industry was enormously stimulated by the discovery, in 1 844, of
OCCURRENCE OF DIAMOND 139
the remarkably rich fields in the State of Bahia, especially at Serra da Cincora, where carbonado, or black diamond, first came to light, but after a- few years, owing to the difficulties of supplying labour, the unhealthiness of the climate, and the high cost of living, the yield fell off and gradually declined, until the importance of the fields was finally eclipsed by the rise of the South African mines. The Brazilian mines have proved very productive, but chiefly in small diamonds, stones above a carat in weight being few in comparison. The largest stone, to which the name, the Star of the South, was applied, weighed in the rough 254^ carats; it was discovered at the Bagagem mines in 1853. The quality of the diamonds is good,