 A pearl is created in the mantle of a mollusc when an irritant particle is surrounded by layers of nacre . Although most bivalves can create pearls , oysters in the family <unk> and freshwater mussels in the families Unionidae and <unk> are the main source of commercially available pearls because the calcareous concretions produced by most other species have no lustre . Finding pearls inside oysters is a very chancy business as hundreds of shells may need to be prised open before a single pearl can be found . Most pearls are now obtained from cultured shells where an irritant substance has been purposefully introduced to induce the formation of a pearl . A " <unk> " ( irregular ) pearl can be grown by the insertion of an implant , usually made of plastic , under a flap of the mantle and next to the mother @-@ of @-@ pearl interior of the shell . A more difficult procedure is the grafting of a piece of oyster mantle into the gonad of an adult specimen together with the insertion of a shell bead nucleus . This produces a superior , spherical pearl . The animal can be opened to extract the pearl after about two years and reseeded so that it produces another pearl . Pearl oyster farming and pearl culture is an important industry in Japan and many other countries bordering the Indian and Pacific Oceans . 
