saltwater v. freshwater pearls

The main different between saltwater and freshwater pearls resides in their production. The nucleus used in all pearls farmed in saltwater today is a mother-of-pearl bead. This bead is composed of oyster shell that has been cut, rounded, and polished. Saltwater oysters are nucleated by opening the shell a mere 2-3 centimeters and making a minute incision to the gonad - the oyster's reproductive organ. The mother of pearl nucleus is inserted into this incision which is then followed with a very small piece of mantle tissue from a donor oyster.  Saltwater oysters will only produce 1-2 pearls per typical nucleation. The Akoya oyster dies at harvest. Tahitian oysters (Pictada margaritifera and Pinctada maxima) accept only one nucleus at a time, but as they do not die at harvest they may be nucleated several times. Freshwater pearls are also nucleated albeit in a different fashion. In lieu of the mother-of-pearl bead freshwater pearl farmers nucleate their mussels with only a piece of mantle tissue. This mantle tissue is not placed in the reproductive organ of the mussel, but in the fleshy mantle tissue. Because the mantle tissue is large and located on either side of the shell, each mussel can withstand up to 50 nuclei at a time.