CHAPTER XL

PEARL, CORAL, AMBER

Although none of the substances considered in this chapter come within the strict definition of a stone, since they are directly the result of living agency, yet pearl at least cannot be denied the title of a gem. Both pearl and coral contain calcium carbonate in one or other of its crystallized forms, and both are gathered from the sea; but otherwise they have nothing in common. Amber is of vegetable origin, and is a very different substance.

PEARL

From that unrecorded day when some scantily clothed savage seeking for succulent food opened an oyster and found to his astonishment within its shell a delicate silvery pellet that shimmered in the light of a tropical sun, down to the present day, without intermission, pearl has held a place all its own in the rank of jewels. Though it be lacking in durability, its beauty cannot be disputed, and large examples, perfect in form and lustre, are sufficiently rare to tax the deepest purse.

The substance composing the pearl is identical with the iridescent lining—mother-o’-pearl or nacre, as it is termed—of the shell. Tortured by the intrusion of some living thing, a boring parasite, a worm, or a small fish, or of a grain of sand or other inorganic substance, and without means to free itself, the mollusc perforce neutralizes the irritant matter by converting it into an object of beauty that eventually finds its way into some jewellery cabinet. Built up in a haphazard manner and not confined by the inexorable laws of intermolecular action, a pearl may assume any and every variety of shape from the regular to the fantastic. It may be truly spherical, egg- or pear-shaped—pear-drops or pear-eyes, as they are termed—or it may be quite irregular—the so-called baroque or barrok pearls. The first is the most prized, but a well-shaped drop-pearl is in great demand for pendants or ear-rings. The colour is ordinarily white, or faintly tinged yellowish or bluish, and somewhat rarely, salmon-pink, reddish, or blackish grey. Perfect black pearls are valuable, but not as costly as the finest of the white. Though not transparent, pearl is to a varying extent translucent, and its characteristic lustre—‘orient’ in the language of jewellery—is due to the same kind of interaction of light reflected from different layers that has been remarked upon in the case of opal and certain other stones. The translucency varies in degree, and some jewellers speak of the ‘water’ of pearls just as in the case of diamonds. If a pearl be sliced across the middle and the section be examined under the microscope, it will be seen that the structure consists of concentric shells and resembles that of an onion. These shells are alternately composed of calcium carbonate in its crystallized form, aragonite, and of a horny organic matter known as conchiolin, and they evidently represent the result of intermittent growth. Because of their composite character, pearls have a specific gravity ranging from 2·65 to 2·69-2·84-2·89 in the case of pink pearls—which is appreciably less than that of aragonite, 2·94: the hardness is about the same, namely, 3½ to 4 on Mohs’s scale. That the arrangement of the mineral layers is approximately parallel is evinced by the distinctness of the shadow-edges shown on examination with the refractometer. Pearls require very careful handling, both because they are comparatively soft and therefore apt to be scratched, and because they are chemically affected by acids, and even by the perspiration from the skin. Acids attack only the calcium carbonate, not the organic matter; the well-known story therefore of Cleopatra dissolving a valuable pearl in vinegar, which is moreover, too weak an acid to effect the solution quickly, must not be accepted too literally. Pearls are not cut like stones, and therefore as soon as the precious bloom has once gone, nothing can be done to revive it. Attempts are sometimes made in the case of valuable pearls to remove the dull skin and lay bare another iridescent layer underneath, but the operation is exceedingly delicate. Even with the best of care pearls must in process of time perish owing to the decay of the organic constituent. Pearls that have been discovered in ancient tombs crumbled to dust at a touch, and those formerly in ancient rings have vanished or only remain as a brown powder, while the garnets or other stones set with them are little the worse for the centuries that have passed by.

The largest known pearl was at one time in the famous collection belonging to the banker, Henry Philip Hope. Cylindrical in form, with a slight swelling at one end, it measures 50 mm. (2 inches) in length, and 115 mm. (4½ inches) in circumference about the thicker, and 83 mm. (3¼ inches) about the thinner end, and weighs 454 carats. About three-quarters of it is white in colour with a fine ‘orient,’ and the remainder is bronze in tint. It is valued at upwards of £12,000. A large pearl, 300 carats in weight, is in the imperial crown of the Emperor of Austria, and another, pear-shaped, is in the possession of the Shah of Persia. A beautiful white India pearl, a perfect sphere in shape, and 28 carats in weight, is in the Museum of Zosima in Moscow; it is known as ‘La Pellegrina.’ The ‘Great Southern Cross,’ which consists of nine large pearls naturally joined together in the shape of a cross, was discovered in an oyster fished up in 1886 off the beds of Western Australia. The collection of jewels in the famous Green Vaults at Dresden contains a number of pearls of curious shapes.

Large pearls are sold separately, while the small pearls known as ‘seed’ pearls come into the market bored and strung on silk in ‘bunches.’ The unit of weight is the pearl grain, which is a quarter of a carat, and the rate of price depends on the square of the weight in grains. The rate per unit or base varies from 6d. to 50s. according to the shape and quality of the pearl. Spherical pearls command the best prices, next the pearl-drops, and lastly the buttons; but whatever the shape, it is imperative that the pearl have ‘orient,’ without which it is valueless. The cheaper grades of pearls are sold by the carat.

For use in necklaces and pendants pearls are bored with a steel drill, and threaded with silk, an easy operation on account of their softness. They harmonize well with diamonds. Small pearls are often set as a frame to large coloured stones, to which they form an admirable foil. Pearls set in rings or anywhere where the upper half alone would show are generally sawn in halves; ‘button’ pearls find an extensive use in modern rings.

Any mollusc, whether of the bi-valve or the uni-valve type, which possesses a nacreous shell, has the power of producing pearls, but only two, the pearl-oyster, _Meleagrina margaritifera_, and the pearl-mussel, _Unio margarifer_, repay the cost of systematic fishing. The outside of the shell is formed of the horny matter called conchiolin; while the inside is composed of two coats, of which the outer consists of alternate layers of conchiolin and calcium carbonate in its crystallized form, calcite, and the inner of the same organic matter, but with calcium carbonate in its other crystallized form, aragonite. The latter coat forms the nacreous lining known as mother-o’-pearl, which is identical in consistency with pearl, but somewhat more transparent. The iridescence of mother-o’-pearl is due not only to the fact that it is composed of a succession of thin translucent layers, but also to the fact that these layers overlap like slates on a house, and form a series of fine parallel lines on the surface; diffraction therefore as well as interference of light takes place, and a similar diffraction phenomenon is displayed even by a cast of the inside of the shell. The animal has the property of secreting calcium carbonate, which it absorbs from the sea-water, in both its crystallized conditions as well as conchiolin. At the outer rim it secretes conchiolin, further in calcite, and at the very inside aragonite. The shape and appearance of a pearl therefore depend on the position in which the intruding substance is situated within the shell. The most perfect pearl has been in intermittent motion in the interior of the mollusc, and has received successive coats according to the position in which it happened to be. A parasite that bores into the shell is walled up at the point of entrance, and a wart- or blister-pearl results. The thinner the successive coats the finer the lustre. Pearls have even been discovered embedded in the animal itself. The number of pearls found in a shell depends on the number of times the living host was compelled to seal up some irritant object, and may vary from one up to the eighty-seven which are said to have been found in an Indian oyster. That an oyster thus distinguished has not led a happy existence is testified by the distorted shape of its shell, a clue that guides the pearl-fishers in their search. Moreover, pearl-oysters never have thick nacreous shells, and on the other hand molluscs with fine mother-o’-pearl seldom contain pearls.

Beautiful white and silvery pearls are found in a small oyster that lives at a depth of 6 to 13 fathoms (11-24 m.) in the Gulf of Manaar, off the coast of Ceylon. About seven-eighths, however, of the pearls that come into the market are obtained from a larger oyster which has its home on the Arabian coast of the Persian Gulf. These famous fisheries have been known since very early times. The pearls found here are more yellowish than those from Ceylon, but are nevertheless of excellent quality. The pearl fisheries off the north-west coast of Western Australia and off Venezuela are also not unimportant, and fine black pearls have been supplied by molluscs from the Gulf of Mexico.

The Chinese have long made a practice of introducing into the shell of a pearl-oyster little tin images of Buddha in order that they may be coated with the nacreous secretion. The Japanese have during recent years made quite an industry of stimulating the efforts of the mollusc by cementing small pieces of mother-o’-pearl to the interior surface of the shell (Plate XXXII, Fig. 1); these ‘culture’ pearls, as they are termed, are recognizable by examination of the back. About a year has to elapse before a coating of a tenth of a millimetre is formed, and another two years must pass before the thickness is doubled. After removal the piece of mother-o’-pearl, which is now coated with several nacreous layers, is cemented to a piece of ordinary mother-o’-pearl, and the lower portion is ground to the usual symmetrical shape (Plate XXXII, Fig. 2). Blister pearls are often similarly treated. In both cases, however, the ‘orient’ is deficient in quality.

The finest mother-o’-pearl is supplied by a mollusc found in the sea near the islands lying between Borneo and the Philippines, and fine material is found at Shark Bay and off Thursday Island.

CORAL

Coral ranks far below pearl and meets with but limited appreciation. It is common enough in warm seas, but the only kind which finds its way into jewellery is the rose or red-coloured coral—the noble coral, _Corallium nobile_ or _rubrum_. It consists of the axial skeleton of the coral polyp, and is built up of hollow tubes fitting one within the other. The composition is mainly calcium carbonate with a little magnesium carbonate and a small amount of organic matter. The former of the mineral substances is in the form of calcite, and the crystals are arranged in fibrous form radiating at right angles to the axis of the coral. The specific gravity varies from 2·6 to 2·7, being slightly under that of calcite, and the hardness is somewhat greater, being about 3¾ on Mohs’s scale.

The best red coral is found in the Mediterranean Sea off Algiers and Tunis in Africa, and Sicily and the Calabrian Coast of Italy. The industry of shaping and fashioning the coral is carried on almost entirely in Italy. Coral is usually cut into beads, either round or egg-shaped, and used for necklaces, rosaries, and bracelets. The best quality fetches from 20s. to 30s. per carat.

AMBER

This fossil resin, yellow and brownish-yellow in tint, finds an extensive use as the material for mouthpieces of pipes, cigar and cigarette-holders, umbrella-handles, and so on, and is even locally cut for jewellery, although its extreme softness, its hardness being only 2½ on Mohs’s scale, quite unfits it for such a purpose. It is only slightly denser than water, the specific gravity being about 1·10. Since the structure is amorphous the refraction is single, the index being about 1·540. Amber, being a very bad conductor of heat, is perceptibly warm to the touch. Its property of becoming electrified by friction attracted early attention, and from the Greek name for it, ἤλεκτρον, is derived our word electricity.

Amber is washed up by the sea off the coasts of Sicily and Prussia, and of Norfolk and Suffolk in England. The finest examples, which are picked up off the shore of Catania in Sicily, are distinguished by a fine bluish fluorescence, resembling that seen in lubricating oil; such pieces command good prices.

A recent resin, pale yellow in colour, known as kauri-gum, is found in New Zealand, where it is highly valued.

TABLES

TABLE I

_Chemical Composition of Gem-Stones_

(_a_) ELEMENTS—

Diamond C

(_b_) OXIDES—

Corundum Al_{2}O_{3} Quartz SiO_{2} Chalcedony SiO_{2} Opal SiO_{2}.nH_{2}O

(_c_) ALUMINATES—

Spinel MgAl_{2}O_{4} Chrysoberyl BeAl_{2}O_{4}

(_d_) SILICATES—

Phenakite Be_{2}SiO_{4} Dioptase H_{2}CuSiO_{4} Peridot Mg_{2}SiO_{4} Zircon ZrSiO_{4} Enstatite MgSiO_{3} Diopside CaMg(SiO_{3})_{2} Nephrite CaMg_{3}(SiO_{3})_{4} Sphene CaTiSiO_{5} Benitoite BaTiSi_{3}O_{9} Andalusite Al(AlO)SiO_{4} Kyanite (AlO)_{2}SiO_{3} Topaz [Al(F,OH)]_{2}SiO_{4} Epidote Ca_{2}(Al,Fe)_{2}(AlOH)(SiO_{4})_{3} Euclase Be(AlOH)SiO_{4} Prehnite H_{2}Ca_{2}Al_{2}(SiO_{4})_{3} Iolite H_{2}(Mg,Fe)_{4}Al_{8}Si_{10}O_{37} { Hessonite Ca_{3}Al_{2}(SiO_{4})_{3} _Garnet_ { Pyrope Mg_{3}Al_{2}(SiO_{4})_{3} { Almandine Fe_{3}Al_{2}(SiO_{4})_{3} { Andradite Ca_{3}Fe_{2}(SiO_{4})_{3} Beryl Be_{3}Al_{2}(SiO_{3})_{6} Spodumene LiAl(SiO_{3})_{2} Jadeite NaAl(SiO_{3})_{2} Moonstone KAlSi_{3}O_{8} Tourmaline{12SiO_{2}.3B_{2}O_{3}.(9-x)[(Al,Fe)_{2}O_{3}].3x[ {(Fe,Mn,Ca,Mg,K_{2},Na_{2},Li_{2},H_{2})O].3H_{2}O Axinite HCa_{3}Al_{2}B(SiO_{4})_{4} Idocrase (Ca,Mn,Mg,Fe)_{2}(Al,Fe)(OH,F)]Si_{2}O_{7}

(_e_) PHOSPHATES—

Beryllonite NaBePO_{4} Apatite Ca_{5}(F,Cl)(PO_{4})_{3} Turquoise CuOH.6[Al(OH)_{2}].H_{5}.(PO_{4})_{4}

TABLE II

_Colour of Gem-Stones_

_Colourless and White._—Diamond, corundum (white sapphire), topaz, quartz (rock-crystal), zircon (when ‘fired’), moonstone; rarely beryl, tourmaline; among the less common species, phenakite, spodumene (colourless kunzite), beryllonite.

_Yellow._—Diamond, topaz, corundum (yellow sapphire), quartz (citrine, Scotch or occidental topaz), tourmaline, zircon, sphene, spodumene, beryl.

_Pink and Lilac._—Corundum (pink sapphire), spinel (balas-ruby), tourmaline (rubellite), topaz (usually when ‘fired’), spodumene (kunzite), beryl (morganite), quartz (rose-quartz).

_Red._—Corundum (ruby), garnet (pyrope, almandine), spinel (balas-ruby), tourmaline (rubellite), zircon, opal (fire-opal).

_Green._—Beryl (emerald, aquamarine), peridot, corundum, tourmaline, chrysoberyl (including alexandrite), zircon, garnet (demantoid); among less common species, spodumene (hiddenite), euclase, diopside, idocrase, epidote, apatite, obsidian; rarely diamond; also semi-opaque, turquoise, jade.

_Blue._—Corundum (sapphire), spinel, topaz, tourmaline, zircon; among the less common species, kyanite, iolite, benitoite, apatite; rarely diamond; also semi-opaque, turquoise, lapis lazuli, sodalite.

_Violet and Purple._—Quartz (amethyst), corundum (oriental amethyst), spinel (almandine-spinel), garnet (almandine), spodumene (kunzite), apatite.

_Brown._—Diamond, tourmaline, quartz (smoky-quartz); among the less common species, andalusite, axinite, sphene.

TABLE III

_Refractive Indices of Gem-Stones_[8]

Opal 1·454 Moonstone 1·53 1·54 Iolite 1·543 1·551 Quartz 1·544 1·553 Beryllonite 1·553 1·565 Beryl 1·578 1·585 Turquoise 1·61 1·65 Topaz 1·618 1·627 Andalusite 1·632 1·643 Tourmaline 1·626 1·651 Apatite 1·642 1·646 Phenakite 1·652 1·667 Euclase 1·651 1·670 Spodumene 1·660 1·675 Enstatite 1·665 1·674 Peridot 1·659 1·697 Axinite 1·674 1·684 Diopside 1·685 1·705 Idocrase 1·714 1·719 Spinel 1·726 Kyanite 1·72 1·73 Epidote 1·735 1·766 Garnet (Hessonite) 1·745 Chrysoberyl 1·746 1·753 Garnet (Pyrope) 1·755 Benitoite 1·757 1·804 Corundum 1·761 1·770 Garnet (Almandine) 1·790 Zircon (a) 1·815 Garnet (Demantoid) 1·885 Sphene 1·901 1·985 Zircon (b) 1·927 1·980 Diamond 2·417

TABLE IV

_Colour-Dispersion of Gem-Stones_[9]

Moonstone ·012 Quartz ·013 Beryl ·014 Topaz ·014 Chrysoberyl ·015 Tourmaline ·017 Spodumene ·017 Corundum ·018 Peridot ·020 Spinel ·020 Garnet (Almandine) ·024 Garnet (Pyrope) ·027 Garnet (Hessonite) ·028 Zircon ·038 Diamond ·044 Sphene ·051 Garnet (Demantoid) ·057

TABLE V

_Character of the Refraction of Gem-Stones_

(_a_) SINGLE—

Diamond, spinel, garnet, opal. Diamond and garnet frequently display local double refraction.

(_b_) UNIAXIAL, POSITIVE—

Quartz ·009 Phenakite ·015 Benitoite ·047 Zircon (b) ·053 Quartz exhibits circular polarization.

(_c_) UNIAXIAL, NEGATIVE—

Apatite ·004 Idocrase ·005 Beryl ·007 Corundum ·009 Tourmaline ·025

(_d_) BIAXIAL, POSITIVE—

Chrysoberyl ·007 Topaz ·009 Enstatite ·009 Spodumene ·015 Euclase ·019 Diopside ·020 Peridot ·038 Sphene ·084

(_e_) BIAXIAL, NEGATIVE—

Moonstone ·006 Iolite ·008 Axinite ·010 Andalusite ·011 Beryllonite ·012 Kyanite ·016 Epidote ·031

TABLE VI

_Dichroism of Gem-Stones_

(_a_) STRONG

Corundum, tourmaline, alexandrite, spodumene, andalusite, iolite, epidote, axinite.

(_b_) DISTINCT

Emerald, topaz, quartz, peridot, chrysoberyl, enstatite, euclase, idocrase, kyanite, sphene, apatite.

(_c_) WEAK

Beryl, diopside.

TABLE VII

_Specific Gravities of Gem-Stones_

Opal 2·15 Moonstone 2·57 Iolite 2·63 Quartz 2·66 Beryl 2·74 Turquoise 2·82 Beryllonite 2·84 Phenakite 2·99 Euclase 3·07 Tourmaline 3·10 Enstatite 3·10 Andalusite 3·18 Spodumene 3·18 Apatite 3·20 Axinite 3·28 Diopside 3·29 Epidote 3·37 Peridot 3·40 Idocrase 3·40 Sphene 3·40 Diamond 3·52 Topaz 3·53 Spinel 3·60 Kyanite 3·61 Garnet (Hessonite) 3·61 Benitoite 3·64 Chrysoberyl 3·73 Garnet (Pyrope) 3·78 Garnet (Demantoid) 3·84 Corundum 4·03 Garnet (Almandine) 4·05 Zircon (a) 4·20 Zircon (b) 4·69

TABLE VIII

_Degrees of Hardness of Gem-Stones_

5. Kyanite (5-7), apatite, lapis lazuli 5½. Enstatite, beryllonite, sphene 6. Opal, moonstone, turquoise, diopside 6½. Spodumene, peridot, garnet (demantoid), benitoite, idocrase, epidote, axinite, jade (nephrite) 7. Iolite, quartz, tourmaline, jade (jadeite) 7¼. Garnet (hessonite, pyrope) 7½. Beryl, garnet (almandine), zircon, phenakite, euclase, andalusite 8. Topaz, spinel 8½. Chrysoberyl 9. Corundum 10. Diamond

TABLE IX.—DATA

_Densities of Water and Toluol at Ordinary Temperatures_

+-----------------------------+----------+----------+ | TEMPERATURE | WATER | TOLUOL | +-----------------------------+----------+----------+ | Centigrade | Fahrenheit | | | | | | | | | 14° | 57·2° | 0·9994 | 0·8697 | | 15° | 59·0° | 0·9992 | 0·8687 | | 16° | 60·8° | 0·9990 | 0·8677 | | 17° | 62·6° | 0·9988 | 0·8667 | | 18° | 64·4° | 0·9986 | 0·8657 | | 19° | 66·2° | 0·9985 | 0·8647 | | 20° | 68·0° | 0·9983 | 0·8637 | | 21° | 69·0° | 0·9981 | 0·8627 | | 22° | 71·6° | 0·9979 | 0·8617 | | 23° | 73·4° | 0·9977 | 0·8607 | +-----------------------------+----------+----------+

1 English carat = 0·2053 gram 1 Metric carat = 0·2000 (one-fifth) gram 1 oz. Av. = 28·35 grams 1 lb. Av. = 0·4536 kilogram 1 inch = 25·4 millimetres 1 foot = 0·3048 metre 1 yard = 0·9144 metre 1 mile = 1·6093 kilometre

INDEX

Absorption, 53, 59

Absorption spectra, 59

Achroite, 220, 221

Adularia, 255

Agate, 247

Akbar Shah diamond, 163

Alalite, 272

Albite, 254

Alexandrite, 54, 60, 233 Scientific, 122

Almandine, 60, 214 Oriental, 112, 172 spinel, 112, 204

Amazon-stone, 255

Amber, 83, 298

Amethyst, 239, 242 Oriental, 111, 172, 239

Anatase, 281

Andalusite, 274

Andradite, 216

Anomalous refraction, 47

Anorthite, 254

Apatite, 279

Apophyllite, 290

Aquamarine, 184, 193

Arizona-ruby, 213

Artificial stones, 124

Asteria, 38, 177

Asterism, 38

Australia stones, 154, 174, 182, 195, 213, 216, 227, 232, 252, 288

Austrian Yellow diamond, 165

Aventurine, 240, 241

Axes, Crystallographic, 9 Optic, 49

Axinite, 278

Azure-quartz, 244

Azurite, 287

Balas-ruby, 203

Barnato, Barnett, 145

Baroque, Barrok, pearls, 292

Bastite, 272

Benitoite, 267

Berquem, Louis de, 90, 161

Beryl, 184

Beryllonite, 270

Bezel facet, 92

Biaxial double refraction, 45, 49, 57

Bisectrix, 45, 49

Black diamond, 129

Black lead, 129

Black opal, 249, 250

Black Prince’s ruby, 206

Blister-pearl, 296

Bloodstone, 247

Blue felspar, 255

Blue ground, 143, 147

Blue John, 285

Boart, 103, 129, 133

Bohemian garnet (pyrope), 207, 212

Bone turquoise, 259

Boodt, A. B. de, 132, 213

Borgis, Hortensio, 161

Borneo stones, 154, 170

Bort, _v._ Boart, 103, 129, 133

Bottle-stone, 284

Boule, 118

Bowenite, 263

Braganza diamond, 170

Brazil stones, 138, 165, 166, 169, 194 _et seq._, 201, 215, 223, 236, 243, 244, 248, 266, 269, 270, 274

Brazilian emerald, 111, 220, 221 peridot, 221 sapphire, 111, 221 topaz, 111, 197

Brilliant form of cutting, 92

Brilliant, Scientific, 122

Bristol diamonds, 243

Bruting, 100

Burma stones, 178, 205, 223, 227, 263

Button-pearl, 295

Byes, Bywaters, 136, 150

Cabochon form of cutting, 88

Cacholong, 251

Cairngorm, 239

Callaica, callaina, callais, 258

Calcite, 40, 289

California stones, 156, 195, 202, 224, 259, 265, 267, 275

Californite, 264, 275

Cape-ruby, 213

Carat weight, 72, 84

Carbon, 129

Carbonado, 129

Carborundum, 105

Carbuncle, 89, 215

Carnelian, 247

Cascalho, 139

Cassiterite, 281

Cat’s-eye (chrysoberyl), 38, 90, 233 (quartz), 39, 90, 240 (tourmaline), 39, 219 Hungarian, 244

Ceylon stones, 181, 195, 201, 205, 212, 215, 216, 223, 232, 236, 237, 243, 244, 255, 267, 274, 279, 284

Ceylonese peridot (tourmaline), 221

Ceylonite, 204

Chalcedony, 246

Chatoyancy, 38

Chert, 247

Chessylite, 287

Chrysoberyl, 233

Chrysocolla, 288

Chrysolite (chrysoberyl), 233 (peridot), 225

Chrysoprase, 247

Church, Sir Arthur, 61, 211, 231

Cinnamon-stone, 211

Citrine, 239

Cleavage, 80, 100, 149

Close goods, 149

Colenso diamond, 131

Colour, 53

Colour dispersion, 20, 97

Conchiolin, 293

Coral, 298

Cordierite, 266

Cornish diamonds, 243

Corundum, 172

Crocidolite, 39, 240

Crookes, Sir William, 132, 153

Cross facet, 93

Crystal, 6, 7, 8 Rock-, 97

Cubic system, 8

Culet facet, 93

Cullinan diamond, 94, 100, 168

Culture pearls, 297

Cumberland diamond, 164

Cyanite (Kyanite), 79, 273

Cymophane, 234

Darya-i-nor diamond, 162

De Beers diamonds, 167

Demantoid, 216

Density, 63

Deviation, Minimum, 30

Diamond, Characters of, 128 cutting, 90 gauges, 86 Glaziers’, 135 mining, 146 Occurrence of, in— Borneo, 154 Brazil, 139 German South-West Africa, 155 India, 138 New South Wales, 154 Rhodesia, 155 South Africa, 139 Origin of, 151 -point, 91 -rose, 92 -table, 91

Diamonds, Classification of, 136, 149 Historical, 157 Prices of, 135

Dichroism, 55

Dichroite, 266

Dichroscope, 55

Diffusion column, 65

Diopside, 272

Dioptase, 280

Dispersion, Colour, 20, 24, 97

Disthene, 273

Dop, 102

Double refraction, 28, 40

Doublet, 125

Dresden diamond, 171

Drop-stone, 94

Duke of Devonshire’s emerald, 191

Edwardes ruby, 175

Electrical characters, 82

Emerald, 89, 184 Brazilian, 220, 221 Evening, 225 Oriental, 111, 172 Scientific, 122 Uralian, 216

Emeraldine, 247

Emery, 175

English Dresden diamond, 166

Enstatite, 271

Epidote, 275

Essence d’Orient, 126

Essonite (Hessonite), 211

Euclase, 269

Eugénie diamond, 164

Evening emerald, 225

Excelsior diamond, 167

Extinction, 45

Faceting machine, 105

False topaz, 239

Felspar, 254

Fire, 20, 96

Fire-marble, 289

Fire-opal, 251

Flats, 150

Flêches d’amour, 240

Flint, 247

Floors, 147

Fluor, 285

Frémy, E., 115

Garnet, 207 Green, 271

Gaudin, M. A. A., 115

Gauges, Diamond, 86

Girdle, 92

Glass, 7, 124

Gnaga Boh ruby, 180

Goniometer, 30

Grain, Pearl, 86

Graphite, 129

Greaser, 149

Great Mogul diamond, 161

Great Southern Cross group of pearls, 294

Great Table diamond, 162

Great White diamond, 167

Green garnet, 271

Greenstone, 261

Grossular, 211

Habit, 12

Hardness, 78

Haüynite, 286

Heavy liquids, 64

Hematite, 282

Hessonite, 211

Hexagonal system, 10

Hiddenite, 266

Hope cat’s-eye, 237 chrysolite, 237 diamond, 170 pearl, 294 sapphire, 121

Hornstone, 247

Hungarian cat’s-eye, 244

Hyacinth, 211, 228

Hydrophane, 250

Hydrostatic weighing, 72

Hypersthene, 271

Iceland-spar, 40, 44

Idocrase, 274

Imitation stones, 124

Imperial diamond, 167

Index of refraction, 16

India stones, 137, 181, 194, 215, 243, 244, 248, 290

Indicators, 65

Indicolite, 221

Interference of light, 39, 48

Iolite, 266

Iris, 240

Isle of Wight diamonds, 243

Isomorphous replacement, 13, 19

Jacinth, 211, 228

Jade, 260

Jadeite, 262

Jargoon, 228

Jasper, 247

Jehan Ghir Shah diamond, 163

Jigger, 149

Jubilee diamond, 167

Kauri-gum, 299

Khiraj-i-Alam ruby, 206

Kimberlite, 152

King topaz, 181, 201

Klein’s solution, 67

Koh-i-nor diamond, 137, 158

Kunz, Dr. G. F., 186, 224, 262, 265

Kunzite, 265

Kyanite, 79, 273

Labradorite, 255

La Pellegrina pearl, 294

Lapis lazuli, 286

Lazurite, 286

Lozenge facet, 93

Lumachelle, 289

Lustre, 37

Maacles, Macles, 12, 150

Madagascar stones, 195, 224, 243, 265, 266

Malachite, 287

Malacolite, 272

Manufactured stones, 113

Marble, 289

Mattan diamond, 155, 170

Matura diamonds, 232

Mazarin, Cardinal, 92

Meerschaum, 288

Mêlée, 136

Methylene iodide, 26, 66

Metric carat, 85, 87

Milky-quartz, 240

Minimum deviation, 30

Mocha-stone, 247

Moe’s gauge, 87

Mohs’s scale of hardness, 78

Moissan, Henri, 153

Moldavite, 283

Monoclinic system, 11

Moon of the Mountains diamond, 162

Moonstone, 39, 255

Morganite, 186, 195

Moroxite, 279

Moss-agate, 247

Mother-of-emerald, 240

Mother-o’-pearl, 292

Nacre, 292

Napoleon diamond, 164

Nassak diamond, 163

Negative double refraction, 45

Nephrite, 261

Nicol’s prism, 44

Nizam diamond, 162

Obsidian, 283

Occidental topaz, 111, 239

Odontolite, 259

Off-coloured diamonds, 130

Olivine (demantoid), 216 (peridot), 225

Onyx, 247

Opal, 39, 249 Fire, 251 -matrix, 251

Opalescence, 39

Optical anomalies, 47

Optic axes, 49

Oriental almandine, 112, 172 amethyst, 111, 172 emerald, 111, 172 topaz, 111, 172

Orient of pearls, 292

Orloff diamond, 160

Orthoclase, 254

Orthorhombic system, 11

Pacha of Egypt diamond, 165

Paste, 47, 124

Paul I diamond, 171

Pavilion, 93

Pavilion facet, 93

Pear-drop pearls, 292

Pear-eye pearls, 292

Pearl, 291 grain, 86 imitations, 126

Pendeloque, 94

Peridot, 225 Brazilian, 221 Ceylonese, 221

Peruzzi, Vincenzio, 92

Phenakite, 269

Pigott diamond, 164

Pipes, 152

Pistacite, 275

Pitt diamond, 100, 159

Plasma, 247, 264

Pleochroism, 57

Pleonaste, 204

Pliny, 6, 88, 138, 184, 191, 241, 249

Polar Star diamond, 163

Polarization, 42

Porter-Rhodes diamond, 166

Positive double refraction, 45

Prase, 240, 247

Prehnite, 278

Pycnometer, 75

Pyrites, 282

Pyrope, 212

Quartz, 50, 238

Quoin facet, 93

Rainbow-quartz, 240

Reconstructed stones, 116

Reef, 144

Reflection of light, 14

Refraction of light, 15

Refractive index, 16

Refractometer, 22, 50

Regent diamond, 100, 159

Retgers’s salt, 69

Rhodes, Cecil J., 145

Rhodesia stones, 155, 183, 213, 236

Rhodolite, 62, 214

Rhodonite, 287

Rock-crystal, 97, 239

Rock-drill, 134

Röntgen rays, 83

Rose form of cutting, 91

Rose-quartz, 240

Rospoli sapphire, 182

Rotation of plane of polarization, 50

Rubellite, 220, 223

Rubicelle, 203

Ruby, 98, 110, 172 Balas-, 203 Cape-, 213

Sancy diamond, 161

Sapphire, 98, 110, 172 Brazilian (tourmaline), 221 -quartz, 244 Water- (iolite), 266 Water- (topaz), 201

Sard, 247

Sardonyx, 247

Saussurite, 263

Schorl, 221

Scientific alexandrite, 122 brilliant, 122 emerald, 122 topaz, 121

Scotch topaz, 239

Seed pearls, 294

Serpentine, 289

Setting of gem-stones, 107

Shah diamond, 163

Sheen, 39

Shepherd’s Stone diamond, 163

Siam stones, 180

Siberia and Asiatic Russia stones, 182, 188, 194, 201, 217, 223, 236, 244, 256, 262, 269, 270, 287

Siberite, 221

Siderite, 244

Silver-thallium nitrate, 69

Skew facet, 93

Skill facet, 93

Smoky quartz, 240

Snell’s laws, 16

Soapstone, 288

Sodalite, 286, 287

Sonstadt’s solution, 67

South Africa stones, 139 _et seq._, 166, 167 _et seq._, 213, 232, 244, 264, 271

Spanish topaz, 239

Specific gravity, 63

Specific-gravity bottle, 75

Spectroscope, 59

Spectrum, 20, 25

Spectrum, Absorption, 59

Spessartite, 216

Sphene, 276

Spinel, 203

Spodumene, 265

Spotted stones, 149

Star-facet, 92

Star of Africa diamond, 168

Star of Este diamond, 165

Star of Minas diamond, 169

Star of South Africa diamond, 141, 166

Star of the South diamond, 139, 165

Starstones, 38, 177

Steatite, 288

Step form of cutting, 98

Stewart diamond, 166

Strass, 124

Sunstone, 255

Synthetical stones, 113

Syriam, Syrian, garnet, 215

Table facet, 92

Table form of cutting, 91

Tavernier, J. B., 91, 129, 137, 161, 162, 170

Templet facet, 92

Tetragonal system, 9

Thulite, 289

Tiffany diamond, 171

Tiger’s-eye, 39, 240

Timur ruby, 206

Titanite, 276

Topaz, 197 Brazilian, 197 False, 239 Occidental, 111, 239 Oriental, 111, 173 Scientific, 121 Scotch, 239 Spanish, 239

Topazolite, 216

Total-reflection, 18, 21

Tourmaline, 43, 219

Trap form of cutting, 98

Trichroism, 57

Triclinic system, 12

Triplet, 126

Turquoise, 257

Turquoise-matrix, 258

Tuscany diamond, 165

Twinning, 12, 47

Uniaxial double refraction, 45, 48 57

Uralian emerald, 217

Uvarovite, 218

Variscite, 259

Verdite, 264

Verneuil, A. V. L., 116

Vesuvianite, 274

Victoria diamond, 167

Violane, 287

Wart-pearl, 296

Water (of diamonds), 129 (of pearls), 292

Water-chrysolite, 284 -sapphire (iolite), 266 -sapphire (topaz), 201

White opal, 249

White Saxon diamond, 165

Wollaston, W. H., 133

X-rays, 83

Yellow ground, 143

Zircon, 228

_Printed by_ MORRISON & GIBB LIMITED, _Edinburgh_

+--------------------------------------------------------------------+ | FOOTNOTES: | | | | [1] The word medium is employed by physicists to express any | | substance 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. | | | | [2] Methylene iodide must be heated almost to boiling-point to | | enable it to absorb sufficient sulphur; but caution must be | | exercised in the 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. | | | | [3] γωνία, angle; μέτρον, measure. For details of the | | construction, adjustment, and use of this instrument the reader | | should refer to textbooks of mineralogy or crystallography. | | | | [4] 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. | | | | [5] 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. | | | | [6] The word paste is derived from the Italian, _pasta_, food, | | being suggested by the soft plastic nature of the material used | | to imitate gems. | | | | [7] Cf. below, p. 149. | | | | [8] The least and the greatest of the refractive indices of | | doubly refractive species are given. | | | | [9] The dispersion is the difference of the refractive indices | | corresponding to the B and G lines of the solar spectrum. The | | value for crown-glass is ·016. | | | +--------------------------------------------------------------------+

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Transcriber’s Notes: - Text enclosed by underscores is in italics (_italics_). - Redundant title page has been removed. - Blank pages have been removed. - Front publication list moved to the back. - Silently corrected typographical errors. - Where possible Unicode fractions have been used, otherwise they are formatted as example “1-5/16”.