Pearl

Pink pearls are occasionally found in the great conch or foun tain shell of the West Indies, Strombus gigas, L.; but these, though much prized, are not nacreous, and their tint is apt to fade. They are also produced by the chank shell, Turbinella scolymus, L. and other gastropods. Yellowish-brown pearls, of little or no value, are yielded by the Pinna squamosa, and bad-coloured con cretions are formed by the Placuna placenta. Black pearls, which are very highly valued, are obtained chiefly from the pearl oyster of the Gulf of Mexico. The common marine mussel Mytilus edulis also produces pearls, which are, however, of little value.

The Pearl

Oyster.—Genuine precious pearls and the most valuable mother-of-pearl are produced by various species and varieties of the genus Margaritifera. The genus is represented in tropical regions in all parts of the world. It belongs to the family Aviculidae, which is allied to the Pectens or scallop shells. In this family the hinge border is straight and prolonged into two auriculae ; the foot has a very stout byssus. Margaritifera is dis tinguished by the small size or complete absence of the posterior auricula. The species are as follows. The type species is M. margaritifera, which has no teeth on the hinge. Geographical races are distinguished by different names in the trade. Specimens from the Malay Archipelago have a dark band along the margin of the nacre and are known as black-edged Banda shell; those from Australia and New Guinea and the neighbouring islands of the western Pacific are called Australian and New Guinea black-lip. Another variety occurs in Tahiti, Gambier islands and Eastern Polynesia generally, yielding both pearls and shell. It occurs also in China, Ceylon, the Andaman islands and the Maldives. An other form is taken at Zanzibar, Madagascar and the neighbour ing islands, and is called Zanzibar and Madagascar shell. Bombay shell is another local form fished in the Persian gulf and shipped via Bombay. The Red sea variety is known as Egyptian shell. Another variety occurs along the west coast of America and from Panama to Vancouver, and supplies Panama shell and some pearls. A larger form, attaining a foot in diameter and a weight of Io lb. per pair of shells, is considered a distinct species, M. maxima. It is the most valuable species of mother-of-pearl oyster.

Dr. Jameson distinguishes in addition to the above 32 species of Margaritifera; all these have rudimentary teeth on the hinge. The most important species is M. vulgaris, to which belong the pearl oyster of Ceylon and southern India, the lingah shell of the Persian gulf and the pearl oyster of the Red sea. Since the open ing of the Suez canal the latter form has invaded the Mediter ranean, specimens having been taken at Alexandria and at Malta, and attempts have been made to cultivate it on the French coast. The species occurs also on the coasts of the Malay peninsula, Australia and New Guinea, where it is fished both for its shells (Australian lingah) and for pearls. Two species occur on the coasts of South Africa but have no market value. M. carchariarum is the Shark's bay shell of the London market. It is taken in large quantities at Shark's bay, Western Australia, and is of rather small value ; it also yields pearls of inferior quality. The pearl oyster of Japan, known as Japan lingah, is probably a variety of M. vulgaris. M. radiata is the West Indian pearl oyster.

The largest and steadiest consumption of mother-of-pearl is in the button trade, and much is also consumed by cutlers for handles of fruit and dessert knives and forks and pocket-knives. It is also used in the inlaying of Japanese and Chinese lacquers, European lacquered papier-mâché work, and trays, &c., and as an ornamental inlay generally. Among the South Sea islands the shell is largely fashioned into fishing-hooks. Among shells other than those of M. margaritifera used as mother-of-pearl may be mentioned the green ear or ormer shell (Haliotis tuberculata) and several other species of Haliotis, besides various species of Turbo.

Origin of Pearls.—While there is no doubt that pearls, whether found in oysters or mussels, are the results of patho logical processes due to irritants, considerable variation of opin ion exists as to the nature of the exciting bodies. As we have seen, the Chinese long ago discovered that river-mussels could be in duced to yield pearls by inserting into them foreign bodies to form matrices for the deposition of pearl matter, and pearl cul ture of this kind has become an industry of importance (see PEARL, CULTIVATED).