 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 Pteriidae and freshwater mussels in the families Unionidae and Margaritiferidae 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 " mabe " ( 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 . 
