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.
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Orignal From: Clams, oysters, mussels and many other
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