Natural pearls are formed when a foreign object enters the shell of a mollusk and irritates the soft mantle tissue within. This irritant can be anything from a minute snail, worm, small shell or sand. This object becomes trapped in a depression in the mantle tissue. This depression deepens until a pouch or sac is formed. The sac separates from the rest of the tissue and nacre-secreting cells within the sac secrete nacre over the irritant. This nacre builds up layer by layer.
Cultured salt water pearls are produced most often by the Akoya oyster from the waters surrounding Japan. The cultured pearl forms because someone inserts both a piece of mantle from a oyster and a nucleus (which is most often a mother-of-pearl bead). Then the oyster coats the nucleus with nacre and the cultured pearl is made like the natural one.
Freshwater pearls of superior quality also come from Japan; more specifically, Japan's largest lake, Lake Biwa. Freshwater pearls are also cultivated in China, Scotland and the United states. The process is the same as salt water but with different mollusks.
Pearls come in a wide range of colors. They should be relatively free from skin blemishes. The more perfectly round the shape the better. The higher the luster, or "orient", the more valuable the pearl is. The larger the pearl is, generally the greater the value.
Some terms and concepts of pearls:
Akoya: Cultured pearls grown primarily in Japan and China.
Blemish: A flaw (scratch or abrasion) on the surface of a gem.
Body color: The basic color of a pearl: white, cream, yellow, pink, silver or black. A pearl can also have a hint of secondary color (see overtone).
Calcium Carbonate: The main mineral component of a pearl's nacre, or outer layer. Mostly calcite and aragonite.
Cultured pearls: Pearls cultivated by artificial insertion of a small bead, often made of mother-of-pearl and mantle tissue, into an oyster. Patented by Kokichi Mikimoto in 1916.
Freshwater pearl: A pearl produced by a fresh water mollusk.
Luster: A combination of the pearl's exterior shine and glow from within. Created by light reflected from tiny crystals in the nacre.
Mabe pearl: Cultured pearls grown against an oyster's shell rather than in an oyster's tissue. The result is a semi-spherical pearl with a relatively flat back.
Mantle: Soft tissue located inside an oyster. Man-made pearls are formed when the mantle surrounds a surgically inserted nucleus.
Mother of Pearl: The iridescent lining of an oyster's shell. Often used as a nucleus for a man-made pearl
Nacre: Pronounced nay-ker. The crystalline substance secreted by an oyster to form a pearl. The microscopic crystals of nacre refract light to produce a pearl's color and luster.
Nucleus: The object which is artificially inserted into a pearl's mollusk during the cultivation process. This becomes the center of a finished pearl.
Overtone: A hint of a secondary color (see body color): pinkish, silverish, or bluish.
Orient: The rainbow effect that seems to encircle a pearl's surface.
South Sea: Large pearls created by tropical oysters. Grown in Australia, Myanmar, Indonesia and the Philippines.
Surface markings: Imperfections on the pearl's surface. Often described as spots, blisters, or indentations
Symmetrical: A pearl with balance of form. Equal or corresponding characteristics on opposite sides.
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