PEARL. Pearls are calcareous concretions of peculiar lustre, produced by certain molluscs, and valued as objects of personal ornament. The experience of pearl-fishers shows that those shells which are irregular in shape and stunted in growth, or which bear excrescences, or are honeycombed by boring parasites, are those most likely to yield pearls.
The substance of a pearl is essentially the same as that which lines the interior of many shells and is known as "mother-of pearl." Sir D. Brewster first showed that the iridescence of this substance was an optical phenomenon due to the interference of rays of light reflected from microscopic corrugations of the sur face—an effect which may be imitated by artificial striations on a suitable medium. When the inner laminated portion of a nacreous shell is digested in acid the calcareous layers are dissolved away, leaving a very delicate membranous pellicle, which, as shown by Dr. Carpenter, may retain the iridescence as long as it is undis turbed, but which loses it when pressed or stretched.
It is obvious that if a pearl presents a perfectly spherical form it must have remained loose in the substance of the muscles or other soft tissues of the mollusc. Frequently, however, the pearl becomes cemented to the interior of the shell, the point of attach ment thus interfering with its symmetry. In this position it may receive successive nacreous deposits, which ultimately form a pearl of hemispherical shape, so that when cut from the shell it may be flat on one side and convex on the other, forming what jewellers know as a perle bouton. In the course of growth the pearl may become involved in the general deposit of mother-of pearl, and be ultimately buried in the substance of the shell. It has thus happened that fine pearls have occasionally been unex pectedly brought to light in cutting up mother-of-pearl in the workshop.
When a pearl oyster is attacked by a boring parasite the mollusc protects itself by depositing nacreous matter at the point of inva sion, thus forming a hollow body of irregular shape known as a "blister pearl." Hollow warty pearl is sometimes termed in trade coq de perle. Solid pearls of irregular form are often produced by deposition on rough objects, such as small fragments of wood, and these, and in fact all irregular-shaped pearls, are termed peals baroques, or "barrok pearls."
A pearl of the first water should possess, in jewellers' language, a perfect "skin" and a fine "orient"; that is to say, it must be of delicate texture, free from speck or flaw, and of clear almost translucent white colour, with a subdued iridescent sheen. It should also be perfectly spherical, or, if not, of a symmetrical pear-shape.
Pearl Fisheries.—The ancients obtained their pearls chiefly from India and the Persian gulf, but at the present time they are also procured from the Sulu seas, the coast of Australia, the shores of Central America and some of the South Pacific islands. The ancient fisheries of Ceylon (Taprobane) are situated in the Gulf of Manaar, the fishing-banks lying from 6 to 8m. off the western shore, a little to the south of the isle of Manaar. The Tinnevelly fishery is on the Madras side of the strait, near Tuticorin. These Indian fishing-grounds are under the control of Government in spectors, who regulate the fisheries. The oysters yield the best pearls at about four years of age. Fishing generally commences in the second week in March, and lasts for from four to six weeks, according to the season. The boats are grouped in fleets of from 6o to 70, and start usually at midnight so as to reach the oyster-banks at sunrise. Each boat generally carries io divers. On reaching the bank a signal-gun is fired, and diving commences. A stone weighing about 40 lb. is attached to the cord by which the diver is let down. The divers work in pairs, one man diving while the other watches the signal-cord, drawing up the sink-stone first, then hauling up the baskets of oysters, and finally raising the diver himself. On an average the divers remain under water from 5o to 8o seconds, though exceptional instances are cited of men remaining below for as long as six minutes. In his work the diver makes skilful use of his toes. To arm himself against the attacks of sharks and other fishes which infest the Indian waters he carries spikes of ironwood; and the genuine Indian diver never descends without the incantations of shark-charmers, one of whom accompanies the boat while others remain on shore. As a rule the diver is a short-lived man.