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. PearlFrom 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, 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 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 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 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 SECTIONS OF CULTURE PEARL 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. CoralCoral 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. AmberThis 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 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. |