All pearls form the same way. An irritant of some sort enters into a mollusk's shell and lodges in its body. To combat the irritation, the mollusk secretes a substance called nacre around the irritant to isolate it. It is this nacre that is applied layer upon layer by the mollusk that forms a pearl.

All pearls up until the beginnings of culturing in the early 20th century were found naturally in nature. Nature still does the work even with cultured pearls, but the irritant is introduced by human hands and the mollusks used to grow the pearl are tended in controlled environments. Cultured pearls are grown in salt water and fresh water:

Saltwater cultured pearls are grown in oysters. Only one pearl is grown per oyster, thus they are more expensive than fresh water pearls. It takes over two years for the oysters to lay enough layers of nacre around the irritant to get a good harvest. Akoya pearls from Japan are arguably the most popular type of salt water pearls, and were the first type of pearl to be cultivated in the early 1920's. They are most usually white with faint rose colored overtones. Other countries that produce cultured pearls are Burma, Philippines, Thailand, Australia and Tahiti. Cultured saltwater pearls are most often white, but also can be cream, pink, yellow, rose, silver, gray and black.

Freshwater pearls are grown in mussels. Twenty or more pearls can be harvested from each mussel, and the time needed to produce a good harvest is less than the saltwater variety. They are generally less expensive than saltwater pearls. Soon after they started saltwater culturing, the Japanese experimented with freshwater culturing. This was done at Lake Biwa in Japan and resulted in pearls that rivaled the saltwater variety, and these pearls appeared in the 1930's. Lake Biwa freshwater pearls were the industry standard until the lake became so polluted in the 1980's that it could no longer be used for pearl farming. China then took up freshwater pearl farming, and after many years of experimenting and experience they became the main producers of high quality freshwater pearls.

While there is no standardized system for grading saltwater or freshwater pearls, value is determined by size, shape, color, luster and surface. Before cultivation and farming, pearls were owned only by the very wealthy. Now their beauty is more available and affordable for all to enjoy.

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