Nowadays freshwater pearls are very common. Freshwater pearls differ from other cultured pearls, in that the great majority of them are not bead-nucleated. Freshwater mollusks are nucleated by creating a small incision in the fleshy mantle tissue of a 6 to 12 month old mussel, then inserting a 3mm square piece of mantle tissue from a donor mussel. Upon insertion, the donor, (graft) tissue is twisted slightly, rounding out the edges. What happens after this point is really just speculation. People think freshwater pearl are formed in deferent ways. Some believe that this tissue acts as a catalyst in producing a pearl sac thus making the 'nucleation' actual 'activation'. Others believe the tissue molds with the host to create a pearl sac, while still others maintain the tissue is the actual nucleus. Although it is said that a freshwater mollusk can withstand up to 25 insertions per valve, it is common industry practice to perform only 12-16 insertions in either valve, for a total production of 24-32 pearls. The mollusks are then returned to their freshwater environment where they are tended for 2-6 years. The resulting pearls are of solid nacre, but without a bead nucleus to guide the growth process the pearls are rarely perfectly round.