Mabé pearls are cultured blister pearls that form on the pearl oyster's shell rather than in its soft tissues. A mold made of plastic or other material inserted between the pearl oyster's shell and body causes the pearl oyster to produce layers of nacre over the mold, which is eventually cut from the shell. These two species of thin-shelled pearl oysters, the most common sources of mabé pearls, also produce free pearls less frequently.