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.