There is also a "natural pearl" and a "cultured pearl." Natural pearls are made by oysters accidentally, and are very rare. Cultured pearls are made by inserting small and a piece of donor mantle tissue (in the case of saltwater pearls) into the reproductive organ of a saltwater oyster, or simply inserting a piece of donor mantle tissue (in the case of freshwater pearls) into a small incision in the mantle of a freshwater mussel.