These pearls are made in pearl oysters – these days the vast majority are cultivated rather than natural, which are extremely rare. Pearl oysters can make a single pearl at a time, unlike their freshwater counterparts which create multiple pearls. Cultivated saltwater pearls are usually a uniform sphere, as the oyster is injected with a small spherical bead, around which the layers of nacre form. See also pearl, cultured seawater pearl.