Cultured saltwater pearls are grown today by pearl producing oysters in several parts of the world, but among the known "white" pearls are the Japanese "Akoya" - the classic round, white pearl. In the 1950´s, cultured saltwater pearls means Japanese "Akoya" pearls, and Mikimoto owned most of the oyster beds accounting for almost 80% of the world´s supply of cultured pearls.
Freshwater cultured peals are grown in the freshwater rather than saltwater, in mussels that live in lakes and rivers. One of the best known freshwater cultured pearls is the Japanese "Biwa" pearl, which is one of the finest and most beatiful of the freshwater pearls.
Freshwater and saltwater pearls differ greatly in value and composition. The three main differences are the culturing process, the nucleus, and the shape.
Freshwater pearls are mantle-tissue nucleated, where saltwater pearls are bead-nucleated. Instead of inserting a mother of pearl bead and a piece of mantle tissue into the gonad of a freshwater mollusk as is the process with an saltwater oyster, only a piece of mantle-tissue is used, and this is inserted into the mantle tissue of the freshwater mollusk, not the gonad. The result is a pearl composed of solid nacre, and the mantle tissue is eventually dissolved or drilled out.
Freshwater pearls are nucleated in the mantle tissue which is on either side of the oyster. This tissue is much larger than the gonad of an saltwater oyster. Therefore the freshwater mollusk can be nucleated up to 25 times on either side, for a total of 50 nucleations. Saltwater oyster, on the other hand, can handle a maximum of 5 nucleations in its gonad, but very rarely is nucleated with more than 2 beads at a time.
Freshwater mollusks are also much easier to farm. The mortality rate is much lower than that of the nucleated Akoya oysters, and fresh water farms rarely deal with natural disasters such as typhoons and red tides that plague Akoya pearl farms.