A pearl created by a pearl farmer under controlled conditions. A pearl is formed when the mantle tissue is injured by a parasite, an attack of a fish or another event that damages the external fragile rim of the shell of a mollusk shell bivalve or gastropod. In response, the mantle tissue of the mollusk secretes nacre into the pearl sac, a cyst that forms during the healing process. By actually inserting a tissue graft of a donor oyster, a pearl sac forms, and its inner side precipitates calcium carbonate in the form of nacre, forming a pearl. This discovery revolutionized the pearl industry, because it allowed pearl farmers to reliably cultivate large numbers of high-quality pearls. In contrast to natural pearls—which have widely varying shapes, sizes, and qualities, and which are difficult to find—cultured pearls could be "designed" from the start to be round and primarily flawless. Cultured pearls can often be distinguished from natural pearls through the use of x-rays, which reveals the inner nucleus of the pearl.