The only real difference between freshwater and saltwater pearls is the species of shellfish they come from. Saltwater pearls from certain kinds of oysters, and freshwater pearls from freshwater mussels. In addition, there are certain differences in the farming techniques for these pearls — saltwater pearls are almost always grown around an implanted bead nucleus, whereas most (but not all) freshwater pearls are grown from only a piece of organic tissue.