There are two types of cultured pearls: saltwater cultured and freshwater cultured.
Round Saltwater Cultured Pearls
A cultured pearl is formed in much the same way as a natural pearl; however, man supplies the initial irritant to get the process started instead of nature. Pearl farmers use a round mother of pearl shell which they insert into the soft tissue of the oyster in an attempt to create a spherical pearl which will possess gem qualities. This implantation operation takes place a few years after the birth of an oyster, when they have grown to a maturity that will be able to accept an intrusion of this size. Not all of the implanting operations will be successful, and some oysters will produce pearls too inferior for jewelry use. Approximately 10% or less of the original crop will be of gem quality. Saltwater cultured pearls are grown over a period of one to two yearsin saltwater, under the constant care of the farmers. The overall quality of the harvest is dependent on a wide variety of factors, including water temperature, water quality and the local food supply for the oysters.
Freshwater Cultured Pearls
Freshwater cultured pearls are produced under controlled conditions in freshwater lakes. The shape of these pearls varies and can include: oval, drop, button, circle (ringed), baroque (irregular) and round.Unlike the saltwater cultured pearl, a piece of mantle tissue from the mussel is used as an irritant to start the process of secreting the nacre. When the mantle tissue decomposes, all that is left at the heart of the pearl is a small cavity. The growth process takes 2 to 6 years. This procedure is less expensive than producing a saltwater cultured pearl.
Keshi pearls are formed when the oyster rejects and expels the implanted nucleus before the culturing process is complete. Because the oyster has expelled the implanted nucleus of the pearl, theresulting keshi pearl is 100% nacre. The keshi pearl is formed of solid nacre; however, it is not classified as a natural pearl because the culturing process is not a natural occurrence.Keshi pearls may form in either saltwater or freshwater oysters. Theyare generally small in size and, because there was no nucleus to guide the ultimate shaping of the pearl, their shapes vary widely. Keshi come in an array of colors, and tend to have a high luster and shimmering surface quality. Most keshi have a greater luster than even the highest quality cultured pearls.