Pearls were the first gems to be used by ancient cultures as treasures and ornaments. Because very little alteration is needed when the pearl comes out of the oyster, the pearl is immediately useful as a beautiful gem.
A pearl is formed when an irritant, such as a tiny parasite or a broken piece of shell, is trapped inside the pearl oyster shell. To soothe the discomfort, the oyster covers the irritant with layers of smooth crystals, called nacre, which gives the pearl its distinctive lustrous look and silky feel. Contrary to popular belief, a single grain of sand trapped inside the shell will not irritate the oyster enough to produce a pearl!
Cultured pearls form exactly the same way, except a small bead, the nucleus, is surgically implanted inside the oyster. It is this nucleus that the oyster covers with its silky nacre to produce a cultured pearl. The only way to tell the difference between a fine gem quality cultured pearl and a natural pearl is to take an X-ray to determine if there is a nucleus present inside the pearl. Most fine pearl jewelry is made from the Japanese Akoya pearl. "Akoya" is derived from the Japanese word for saltwater, which is where the pearl oyster lives. Saltwater pearls are now cultured in other regions, particularly China, but the quality is not yet as good as the Japanese pearls.
Freshwater cultured pearls are grown in clams or muscles in rivers and lakes around the world. While they are usually large enough to hold several pearls, they will not tolerate the presence of a hard nucleus. Instead, a small piece of mantle tissue, which is the actual flesh of another clam or muscle, is inserted into the shell and the pearls which form follow the curves of these softer centers. This results in the variety of different shapes and textures which is common with freshwater pearls.
Mabe pearls are large hemispherical cultured pearls that are grown against the inside of the pearl oyster shell. They are less expensive than round cultured pearls, and because they are hemispherical, they must be mounted flat against a plastic or mother-of-pearl backing and are used mainly in rings and earrings.
South Sea and Tahitian cultured pearls are the largest of pearls. They can range in size from 8mm to almost 20mm. The nicer quality larger pearls are extremely rare and expensive. The pearls are formed with a nucleus just like cultured pearls, however a larger type of pearl oyster is used. This larger oyster grows well in the warmer South Pacific Ocean and is able to produce the larger sizes of pearls.
Keshi Pearls are probably the most obscure and misunderstood pearl on the market today. Not many people realize that the Keshi is actually a seedless pearl and is found when harvesting cultured pearls. They are actually formed as a byproduct of the culturing process for every type of cultured pearl, whether akoya, South Sea, Tahitian or freshwater.
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