Pearl Jewelry

Pearls, lustrous concretion produced by certain bivalve mollusks and valued as a gem. Pearls consist almost entirely of nacre, which is the substance forming the inner layers of the mollusk shells. Nacre, known as mother-of-pearl, is composed primarily of aragonite crystals. The pearl is an abnormal growth resulting from the invasion of the body of the mollusk by a minute irritant, such as a parasite. The particle acts as an irritant in the mollusk and becomes coated with layer upon layer of nacreous material. Both marine and freshwater mollusks produce pearls, but the most valuable varieties originate in the pearl oyster of the Persian Gulf.

The most highly prized pearls are spherical. When a pearl that has been cut from the shell presents a hemispherical surface, it is sometimes called a bouton pearl. If a solid pearl has an irregular shape, having grown over a rough object, it is known as a baroque pearl. In the jewelry trade, pearls are commonly known as pear, bell, or drop, according to the shape. Pearl coloration varies widely, the most prized shades being white, black, rose, and cream.

One of the important marine-pearl fisheries on the North American continent is off Baja California, Mexico, the central point being at La Paz. Other fisheries are located in the Gulf of Panama, in the West Indies and the islands of the South Pacific, and along the coasts of India, the Persian Gulf, Japan, Mexico, and western Central America, especially in the Pearl Cays, near the coast of Nicaragua. In Australian waters pearls are fished on the coast of Western Australia and of Queensland and in Torres Strait.

River pearls are produced by freshwater mussels in various parts of the world. China is the principal trader in river pearls.

Natural, spherical pearls have been cultured successfully since 1920. In this process a mother-of-pearl bead, from three-quarters to nine-tenths of the diameter of the desired product, is introduced into the pearl oyster. Over a period of years the oyster deposits layers of nacre around the bead. Cultured pearls are not easily distinguished from genuine pearls except by an expert. The technique of producing spherical cultured pearls was developed in Japan, and the culturing of pearls is a major Japanese industry. Artificial pearls, in contrast to cultured pearls, are entirely artificial, made largely of glass.