The mollusk surrounds the irritation with nacre, a secretion from the mantle. This is the same substance that lines the oyster's shell and is called mother-of-pearl. Many thin layers of nacre give a pearl its luster. The result is a sore spot for the oyster but a beautiful jewel for man.
The biggest pearl oysters are found in the South Seas. The Persian Gulf yields a yellowish pearl. Some may be pink, bluish, gray, or black. The coasts of Australia, Venezuela, Malaya, Mexico, and lower California are other important sources of pearls. Many mollusks produce pearls, but only two types produce precious pearls. These are the genera Meleagrina of the tropical seas and Unio of fresh water streams.
The average pearl takes about seven years for its development. Its value is determined by its size and luster. The largest pearl found was about two inches in diameter. Cultured pearls are real pearls but the original nucleus was inserted by man. They are not as costly as true pearls.
Unfortunately, pearls are perishable. Sun-light and skin acids are injurious to them. Pearls should be kept clean and wrapped in moist coverings when not in use. With care, they last over a hundred years.
