Pearl farms vary greatly depending on the type of pearls being produced. One commonality all pearl farms share, however, is the nucleus. Every pearl produced commercially today, except Keshi pearls and pearls from the Bahrain , have been nucleated.
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. A nucleus of high quality will be white without a lot of calcium carbonate streak that may show through the pearl nacre. This is the most common nucleus used today, although others have been utilized to some extent with limited success.
Saltwater oysters are nucleated by opening the shell a mere 2-3 centimetres 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. The mantle tissue is placed between the mother of pearl bead and the gonad with the side containing epithelial cells facing the nucleus.These epithelial cells are the catalyst of the pearl-sac. The pearl sac grows around the nucleus and begins to deposit nacre.
Saltwater oysters will only produce 1-2 pearls per typical nucleation. Akoya oysters can be nucleated with up to 5 beads, but the use of only 2 is most common.The Akoya oyster dies at harvest. South Sea and Tahitian oysters (Pinctada 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. If a particular oyster has been successfully nucleated several times and consistently produces fine pearls, the oyster is often returned to the wild to strengthen the genes of future generations.
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.
The large number of freshwater pearls produced per mussel accounts for some of the diminished value between freshwater pearls and their saltwater cousins, but because the mantle tissue is dissolved into the pearl-sac, freshwater pearls are solid nacre.