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A cultured pearl is a pearl created by a pearl farmer under controlled conditions. Cultured pearls can be farmed using two very different groups of bivalve mollusk: the freshwater river mussels, and the saltwater pearl oysters. ==Development of a pearl== A pearl is formed when the mantle tissue is injured by a parasite, an attack of a fish or another event that damages the external fragile rim of the shell of a mollusk shell bivalve or gastropod. In response, the mantle tissue of the mollusk secretes nacre into the pearl sac, a cyst that forms during the healing process. Chemically speaking, this is calcium carbonate and a fibrous protein called conchiolin. As the nacre builds up in layers of minute aragonite tablets, it fills the growing pearl sac and eventually forms a pearl. Natural pearls are formed by nature, more or less by chance. On the other hand, cultured pearls are human creations formed by inserting a tissue graft from a donor oyster, upon which a pearl sac forms, and the inner side precipitates calcium carbonate, in the form of nacre or "mother-of-pearl". The most popular and effective method for creating cultured pearls are made from a the shells of freshwater river mussels harvested in the midwestern states of the U.S., from Canada to the Gulf of Mexico. Shells with the common names, "Washboard" "Maple Leaf" "Ebony" "Pimpleback" and "Three Ridge" are popular for use in pearl culture due to their compatibility with the host animal, and the nacre they are to be covered by. These high-quality and sought-after shells are first sliced into strips and then into cubes. The edges and corners are ground down until they are a roughly spherical and then milled to become perfectly round, and brought to a highly polished finish. After the nucleus is ready, the next step is obtaining the mantle tissue. The mantle tissue is harvested from one oyster and cut into small pieces. After obtaining the mantle tissue from the first oyster it is time to operate on the second animal. The oyster is placed in warm water to relax the animal. Then it is gently pried open and mounted in a stand to be operated on. A small incision is made and the nucleus is inserted along with a small piece of mantle gland. The oyster is then placed back in the water and allowed over several years to coat the nucleus with nacre. The nucleus is coated in many layers of this nacre, so that when pearls are cut in half, visible layers can be seen. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「cultured pearl」の詳細全文を読む スポンサード リンク
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