pearl ash: see potassium carbonate.

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, 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.

mother-of-pearl or nacre, the iridescent substance that forms the lining of the shells of some fresh-water and some salt-water mollusks. Like the pearl it is a secretion of the mantle, composed of alternate layers of calcium carbonate and conchiolin. Among the chief sources are the pearl oyster, found in warm and tropical seas, chiefly in Asia; freshwater pearl mussels, which live in many rivers of the United States, Europe, and Asia; and the abalone of California, Japan, and other Pacific regions.

Pearl, The, one of four Middle English alliterative poems, all contained in a manuscript of c.1400, composed in the West Midland dialect, almost certainly by the same anonymous author, who flourished c.1370-1390. The Pearl is usually explained as an elegy for the poet's young daughter; in an allegorical vision of singular beauty he sees her as a maiden in paradise and becomes reconciled to her death. The second and third poems, Cleanness (or Purity) and Patience, are homiletic poems on those virtues. Sir Gawain and the Green Knight, the fourth poem, which relates a fabulous adventure of Gawain, is perhaps the most brilliantly conceived of all Arthurian romances. If single authorship is accepted, the artistry displayed in this poem and in The Pearl make the so-called Pearl-poet in some respects a rival to Chaucer. A fifth poem, St. Erkenwald, is attributed by some authorities to the same anonymous author.

Pearl River, uninc. village (1990 pop. 15,314), Rockland co., SE N.Y., near the N.J. line. It is a residential suburb of New York City, and a computer and telecommunications research and development center.

Pearl Harbor, land-locked harbor, on the southern coast of Oahu island, Hawaii, W of Honolulu; one of the largest and best natural harbors in the E Pacific Ocean. In the vicinity are many U.S. military installations, including the chief U.S. Pacific naval base, Hickam Air Force Base, Pearl Harbor Naval Air Station, and Camp H. M. Smith, headquarters of the U.S. Pacific Command.

The United States first gained rights there in 1887, when the Hawaiian monarchy permitted a coaling and repair station. After the United States annexed Hawaii in 1900, Pearl Harbor was made a naval base. Harbor improvements and fortifications were later added, especially after the signing of the Berlin Pact in 1940 by the Axis nations.

On Dec. 7, 1941, while negotiations were going on with Japanese representatives in Washington, Japanese carrier-based planes swept in without warning over Oahu and attacked (7:55 A.M. local time) the bulk of the U.S. Pacific fleet, moored in Pearl Harbor. Nineteen naval vessels, including eight battleships, were sunk or severely damaged; 188 U.S. aircraft were destroyed. Military casualties were 2,280 killed and 1,109 wounded; 68 civilians also died. On Dec. 8, the United States declared war on Japan.

There were many charges of negligence against those responsible for Pearl Harbor's defense. A special commission appointed by President Franklin D. Roosevelt accused the army and navy commanders at Hawaii of dereliction of duty in a report on Jan. 24, 1942. Later army and navy investigations concluded that no valid grounds existed for court-martial. A congressional committee, formed in Sept., 1945, absolved the army and navy commanders in a formal report on July 16, 1946, but censured the War Dept. and the Dept. of the Navy.

Pearl Harbor is now a national historic landmark; a memorial has been built over the sunken hulk of the USS Arizona. The battleship Missouri, site of Japan's surrender, is also preserved there as a memorial.

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, 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.

Lang, Pearl, 1922-, American dancer and choreographer, b. Chicago. Lang was a soloist with Martha Graham's company (1942-52) before forming her own company in 1952. As a dancer and choreographer she was noted for her range of dramatic expression in such works as Rites (1953) and Falls the Shadow Between (1957). Lang appeared in Broadway musicals, including Carousel and Finian's Rainbow. She taught at the Yale Univ. School of Drama and the Juilliard School of Music.

Buck, Pearl Sydenstricker, 1892-1973, American author, b. Hillsboro, W.Va., grad. Randolph-Macon Women's College, 1914. Pearl Buck was awarded the 1938 Nobel Prize in Literature. Until 1924 she lived principally in China, where she, her parents, and her first husband, John Lossing Buck, were missionaries. She is famous for vivid, compassionate novels about life in China. The Good Earth (1931; Pulitzer Prize), considered her finest work, describes a Chinese peasant's rise to wealth and brilliantly conveys a sense of the daily life of ordinary Chinese people. Among her other novels of China are East Wind: West Wind (1930), Dragon Seed (1942), Imperial Woman (1956), and Mandala (1971). In 1935, she married her publisher Richard J. Walsh, president of the John Day Company. In 1949 she founded Welcome House, which provided care for the children of Asian women and American soldiers; the Pearl Buck Foundation of Philadelphia, to which she consigned most of her royalties, aids in the adoption of Amerasian children. Her more than 85 books include works for children, plays, biographies, and works of nonfiction, such as China As I See It (1970).

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.

Learn more about pearl with a free trial on Britannica.com.

River, central Mississippi, U.S. It flows southwest through Jackson, then into Louisiana and the Gulf of Mexico. About 410 mi (660 km) long, it forms the boundary between Mississippi and Louisiana. Honey Island Swamp, lying in the mid-delta area southwest of Picayune, Miss., is noted for its wildlife and fishing.

Learn more about Pearl River with a free trial on Britannica.com.

Inlet, southern coast of Oahu island, Hawaii, U.S. It lies 6 mi (10 km) west of Honolulu, forming a landlocked harbour connected with the Pacific Ocean. In 1887 Hawaii granted the U.S. the exclusive use of the harbour as a coaling and repair station, and in 1908 a naval station was established. In 1941 the harbour was attacked without warning by the Japanese air force, causing great loss of life and precipitating U.S. entry into World War II. It is now the headquarters of the U.S. Pacific Fleet.

Learn more about Pearl Harbor with a free trial on Britannica.com.