Cultured freshwater pearls differ from natural pearls (their growth is initiated by humans : freshwater mollusks are nucleated by creating a small incision in the fleshy mantle tissue and inserting a piece of mantle tissue from another oyster), while natural pearls are formed randomly in nature. Cultured freshwater pearls are composed entirely of a high quality, thick nacre. They are noted for their wide range of colors and shapes.
They are producted from several bivalve mollusks in China (first producer) and in Japan (Biwa Lake).