You may have seen the term cultured" preceding the word pearls at your jewelry store. Cultured pearls are pearls that are nudged to life when a worker surgically implants a tiny bead into the oyster (that's the shellfish in which pearls grow). The host oyster is then lowered back into the water and, if all goes well, it deposits layer upon layer of a substance called nacre around the bead, eventually forming a pearl large enough to harvest. Of course, some oysters continue to produce pearls without any help, forming nacre around a natural irritant that gets inside their shells. They are rare however. Culturing produces far more pearls than nature could alone.