pearl, gem

pearl, hard, rounded secretion formed inside the shell of certain mollusks, used as a gem. It is secreted by the epithelial cells of the mantle, a curtain of tissue between the shell and body mass, and is deposited in successive layers around an irritating object—usually a parasite in the case of natural pearls—that gets caught in the soft tissue of the mollusk. The pearl is built up of layers of aragonite or calcite (crystalline forms of calcium carbonate) held together by conchiolin (a horny organic substance); its composition is identical to that of the mother-of-pearl mother-of-pearl or nacre (nā`kər)
..... Click the link for more information. , or nacre, that forms the interior layer of the mollusk shell.

Pearls may be rice-shaped, round, pear-shaped, button-shaped, or irregular (baroque) and are valued in that order. Pearls found attached to the inner surface of the shell are known as blister pearls. The best pearls are usually white, sometimes with a creamy or pinkish tinge, but may be tinted with yellow, green, blue, brown, or black. Black pearls, because of their rarity, are often highly valued. The unique luster, or orient, of pearls depends upon the reflection and refraction of light from the translucent layers and is finer in proportion as the layers are thinner and more numerous. The iridescence which some pearls display is caused by the overlapping of successive layers, which breaks up light falling on the surface. Pearls are not cut or polished like other gems. They are very soft and are injured by acids and heat; as organic products, they are subject to decay.

Commercially valuable pearls are obtained from pearl oysters (especially of the genus Pinctata) and from freshwater pearl mussels (especially of the genus Hyriopsis). The largest natural pearl center is the Persian Gulf, which is said to produce the finest saltwater pearls. Other important sources are the coasts of India, China, Japan, Australia, the Sulu Archipelago, various Pacific islands, Venezuela, and Central America, and the rivers of Europe and North America. In ancient times the Red Sea was an important source.

Nearly all of the world's supply of cultured saltwater pearls is produced by the Japanese, who have perfected the techniques of saltwater pearl cultivation. These pearls are commonly produced by placing a small mother-of-pearl bead enclosed in a piece of mantle tissue in the body of the oyster. The oysters are then placed in cages that are suspended into sheltered bays for the period of time (up to 4 years) required for pearl formation.

Freshwater pearl cultivation is dominated by the Chinese. To produce freshwater pearls, a small piece of mantle tissue from one mussel is placed into a second mussel; shell beads and small seed pearls are also used. The quantity of freshwater pearls produced far exceeds that of saltwater pearls, and freshwater pearls are also significantly cheaper. Although many freshwater pearls are irregular oblong "rice pearls," round and near-round pearls are also produced. Inferior Chinese pearls are crushed and used in cosmetics and medicines.

Bibliography

See N. Landman et al., Pearls (2001).

Pearl, river, China

Pearl, Chin. Zhujiang, river, 110 mi (177 km) long, S Guangdong prov., S China. Formed at Guangzhou by the confluence of the Xi and Bei rivers, it flows E then S past Guangzhou and Huangpu island to form a large estuary between Hong Kong and Macao. The river links Guangzhou to Hong Kong and the South China Sea and is one of China's most important waterways and one of the centers of its world trade. It is vitally important to the special economic zones that lie along its estuary. The estuary, called Boca Tigris, is kept open for ocean vessels by dredging.

Pearl, river, United States

Pearl, river, 485 mi (781 km) long, rising in E Miss. and flowing S to Lake Borgne, an inlet of the Gulf of Mexico; its lower section (116 mi/187 km) forms the Miss.-La. boundary. Above Jackson, Miss., the Pearl's largest city, is Ross Barnett Reservoir, one of the state's chief water-storage areas.

pearl

Blister pearl attached to the shell of an Oriental pearl oyster (Pinctada martensii Dunker)

(credit: Courtesy of The American Museum of Natural History, New York)

Concretion formed by a mollusk and consisting of the same material (called nacre, or mother-of-pearl) as the mollusk's shell. Long treasured as gemstones, pearls are valued for their translucence and lustre and for the delicate play of surface colour. The more perfect a pearl's shape and the deeper its lustre, the greater its value. The colour varies with the mollusk and its environment. Jewelers of the 16th–17th centuries often used irregularly shaped (baroque) pearls, formed from muscular tissue, to form the bodies of animals and other figures. In Europe and China, mother-of-pearl has been used as an inlay material for decorating furniture. The discovery that a pearl could be cultivated by insertion of a foreign object inside the mollusk's shell is said to have been made in 13th-century China.