Forest Preserve District of Cook County, Illinois
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Nature Bulletin No. 614-A   October 23, 1976
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

About ten years ago, a large valuable pearl was found in a mussel taken from the Wabash River. In years past, news of such finds often started outbreaks of "pearl fever. " Then, people flocked to rivers and creeks where they waded, dived, raked and dredged for mussels. 'These were broken open, searched for that dreamed of pearl and tossed aside. Some were successful. For instance, in the years 1889 to 1897 a quarter of a million dollars worth of pearls were sold from the Mackinaw River, a small stream in central Illinois. In recent decades, however, these slow- growing mollusks have become scarce due to exploitation and increasing pollution. Now, the discovery of a freshwater pearl is a rare event.

Clams, oysters, mussels and many other mollusks with limy shells are known to produce pearls. but very few kinds yield gem pearls of jeweler's quality. The pearl is an abnormal growth of mother-of-pearl, or nacre, imbedded in the soft bodies of these shellfish. It is built up, layer upon layer, in the same way as nacre is added to the lining of the growing shell and always has the same color and luster. For example, over the country, hundreds of good-sized pearls are found each year in the oysters we eat. Unfortunately these have no commercial value regardless of whether they have been cooked or not because they are dull opaque white or purple like the shell of the parent oyster. In recent times almost all pearls of gem quality come from the oriental pearl oyster which has a bright shimmering translucent nacre.

A pearl starts growing when some irritating foreign substance such as a sand grain, bit of mud, parasite or other object becomes lodged in the shell-producing gland called the mantle. Pearls formed in the soft flesh where nacre can be added on all sides are most likely to be spherical and the most highly prized. By far the great majority are flattened or variously distorted and have little value. Size, color, luster and freedom from flaws are other essential qualities. Unlike other gems, such as diamonds, pearls have an average life of only about 50 years. In time the small amount of water in a pearl's make-up is lost and its surface cracks. Because they are mostly lime, necklaces which are worn often are injured by the acid secretions of the human skin.

Centuries ago the Chinese learned that beads or tiny figures of Buddha slipped between the soft mantle and the shell of a living clam became coated with a blister of mother-of-pearl and could be sold as a curio or religious object. With this as a hint a Japanese pearl hunter named Mikimoto began to cut mother-of-pearl beads and insert them between the mantle and shell of pearl oysters and return them to an ocean bay near his home. Many of these grew in size and took on the sheen and beauty of true pearls but, because they were in contact with the mantle on only one side, they became half-pearls with little value as jewelry.

After years of experimenting, Mikimoto finally learned how to grow spherical gem pearls and founded the great cultured pearl industry of Japan. Briefly, this is the method. A mother-of-pearl bead is wrapped in a piece of the living mantle of one pearl oyster and is skillfully transplanted into the flesh of another. Women divers care for the oysters by the tens of thousands in special floating cages. After three or more years, a small percentage of them yield pearls no less beautiful than the best "wild" pearls.

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