A pearl is formed when an irritant, such as sand or a parasite, becomes lodged in the shell of an oyster which lives in saltwater, or a mollusks which lives in freshwater lakes and rivers. The oyster/mollusks deposits layers of a semi-translucent crystalline material called "nacre" around the intruder, where it builds up in layers like the rings of a tree. This process of building up can continue for years, resulting in a pearl. In nature, pearls take many years to develop and often have irregular shapes, ranging from slightly off-spherical to twisting, bulging shapes called "baroque." In any shape, natural pearls are rare and very costly.
The pearl formed inside an oyster is called Saltwater Pearl, while the pearl formed inside a mollusks is called Freshwater Pearl.
Freshwater pearls are often somewhat less lustrous than their saltwater counterparts. However, they appear in a wide variety of shapes and colors, and they tend to be less expensive than saltwater pearls, making them quite popular. Freshwater pearls are also quite durable, resisting chipping, wear, and degeneration.
Recently, techniques have been developed to culture pearls in freshwater mollusks. These pearls are sometimes called "Biwa pearls" after the lake in Japan where they were first developed, but today that name should only be used for cultured pearls from Lake Biwa.
Freshwater cultured pearls are cultivated around the world, including China, Japan and USA, and come in a variety of shapes and sizes. Some are spheres and used like other round pearls; some are buttons and used to accent jewelry; and some are blister shapes called Mabé which are often used in earrings.
