[O.E. perle.] A white, hard, smooth, shining substance, found in some shell-fish, especially in the pearl - oyster, river-mussel, and certain univalves. It is highly valued for its beauty, and ased as a jewel. The shells are lined with a secretion of extremely thin semi-transparent nlnis, and in due time layers of considerable thickness are formed, which gradually harden into the material known by the name of nacre, or mother-of-pearl. Besides the pearl lining of the shells, rounded portions of nacre or mother-of-pearl are found in the flesh of the oyster. These are supposed to be formed by the intrusion of some foreign body, such as a grain of sand, around which layers of nacre are deposited one after another, as many as from ten to twelve round pearls of different sizes being often found within one shell. The best pearls are found off the coast of Ceylon, in the Persian Gulf, on the coast of Australia, and on the Pacific coast of America. The pearl-fishing season lasts from Four to six weeks. A fleet of about 250 boats is engaged in the fishery, each boat having a crew of thirteen men and ten divers, five of the latter being employed in diving whilst the other five are resting. The work is done very rapidly, as the divers cannot remain much more than a minute under water. Each diver is let down from the boat by a rope, weighted with a stone. The usual depth is from 60 to 70 feet. The most valuable pearls are those which are perfectly round ; but these are very scarce, and secure high prices. They are used to form the centre of necklaces. Pearls have been prized as articles of decoration and ornament in all ages of the world. Cleopatra is said to have owned two very large and beautiful ones. Many splendid pearls are owned by the different crowned heads of Europe ; but the Shah of Persia is said to have the finest.