Blister pearls occur when an irritant gets trapped against the oyster shell beneath the mantle tissue. The mollusk then secretes layers of nacre that covers the irritant to form a hemispherical pearl with a flat back. Designers make jewelry pieces by cutting out the blister pearl while it is still attached to the shell, or by cutting the pearl from the shell and filling the back with resin and capping it with a flat piece of nacre (mother-of-pearl). When the latter is done the finish piece is called a mabe pearl or pearl doublet.