Pearl, a hard, lustrous mass formed in the living body of certain shellfish. Pearls of fine quality are gems ranking in desirability and price with rubies, diamonds, and emeralds. Pearls are perhaps the oldest gem valued by man, and have been a part of history and legend for 4,000 years. The pearl has long been a symbol of purity and perfection. The custom of giving pearls to brides and of ornamenting bridal costumes with pearls probably comes from the Hindu legend that the god Krishna gave his daughter a pearl to wear on her wedding day.

Qualities of Pearls

A pearl is considered to be of gem quality when it meets certain standards in regard to shape, size, luster, iridescence, and freedom from blemishes. Round pearls and symmetrical drop- and pear-shaped ones are more valuable than irregularly shaped, or baroque, pearls. A perfect small round pearl is of greater value than a larger blemished or baroque pearl.

Pearls are usually white with a creamy or rosy tint. They may also be tinted with other colors, such as green, blue-gray, and brown. Black pearls, because of their rarity, are highly valued. Color, however, has less to do with value than does orient, the combination of luster and iridescence produced by light striking through the translucent pearl.

Pearls are weighed in grains. (One grain equals one-fourth of a carat, or .05 gram; there are 567 grains to an ounce.) Pearls weighing less than one-half grain are called seed pearls. The largest gem pearl known to exist is the Hope Pearl, a baroque pearl weighing 1,860 grains.

How Pearls Are Formed

Pearls are found only in bivalve mollusks (a kind of shellfish) that line their shells with nacre. Nacre consists chiefly of thin layers of aragonite, a hard, lustrous form of calcium carbonate. It also contains small amounts of calcite, a less lustrous form of calcium carbonate, and a gluelike protein called conchiolin.

Pearls, as well as the shell lining (mother-of-pearl), are made of nacre. Nacre is damaged by contact with the acids in human skin, and, contrary to popular opinion, pearls keep their beauty longer if they are not frequently worn.

Pearl-producing mollusks are popularly called pearl oysters, but true oysters do not produce nacre. Several genera of mollusks form pearls; the finest gems come from the genus Pinctada.

Natural Pearls

When an irritant, such as a tiny parasite or a grain of sand, enters the mollusk's mantle (body wall), certain cells of the mantle secrete nacre to enclose the irritating substance and protect surrounding tissues. After about three years, the layers of nacre form a pearl of marketable size.

Cultured Pearls

are produced more quickly than natural ones by a combination of natural processes and human skills. A mother-of-pearl bead is inserted into a small bag of mantle tissue from one pearl oyster and the bag is inserted into the mantle of another. The seeded mollusks are then put in wire baskets suspended in the water from wooden rafts or plastic buoys.

After a period of 18 months to 3 years the resulting pearl is removed and polished. A cultured pearl usually has an orient inferior to that of a natural pearl because it has fewer (and thicker) layers of nacre, and therefore less luster and iridescence. The best cultured pearls, however, can be distinguished from natural ones only by an X-ray examination.

Imitation Pearls

are made by dipping glass beads repeatedly into pearl essence, a mixture made from fish scales. They can be distinguished from natural and cultured pearls by the visible layers of loose, dried pearl essence surrounding the core.

The Pearl Industry

The best pearls are found in tropical salt waters. The Persian Gulf, the Bay of Bengal, gulfs and inlets of the South China Sea, and the Celebes Sea are noted for their yield of fine pearls. Pearls of gem quality are also taken from the Pacific Ocean off the shores of China, Japan, Australia, eastern Africa, Mexico, and Central America. The Caribbean Sea and the Gulf of Mexico yield valuable pearls.

The largest pearl fisheries and markets are in Asia. Pearl oysters, formerly harvested by divers, are now collected with long rakes or tongs.

Freshwater mollusks, primarily mussels, may produce pearls of value. In the United States, there are pearl fisheries on the banks of the Mississippi River and its tributaries. However, most mollusks harvested in this area are taken for their mother-of-pearl, which is used chiefly to make cores for cultured pearls.

The cultured pearl industry was developed in the 20th century. The Chinese learned to make cultured pearls in the 13th century, but the pearls were not round. Methods of producing round, unblemished pearls were discovered by the Japanese, and Japan is the center of the industry.