The term pearl is applied properly to any mollusk-formed calcareous concretion that displays an orient and is lustrous. There are two major groups of bivalved mollusks in which gem pearls may form: the salt-water pearl oyster (Pinctada), of which there are several species; and a number of genera of fresh-water clams. Usually, jewelers refer to salt-water pearls as Oriental pearls, regardless of their place of discovery, and to those from freshwater bivalves as freshwater pearls.