In full "mabe pearl". A type of large, usually hemispherical cultured pearl, grown on the inside shell of an oyster rather than within the body, and often (because of its shape) mounted in jewellery.
Originally produced in Japan, mabe pearls are now cultured throughout the Pacific Rim, principally grown in the wing oysters Pteria penguin and P. maxima.
1940s; earliest use found in The New York Times. From Japanese mabe the bivalve Pteria penguin, a pearl cultivated in the shell of this bivalve.