A CALCAREOUS spheroid secreted by the mantle of a mollusk (mostly bivalves, but some gastropods too), in layers, to reduce the irritation of some foreign particulate matter lodged within the tissues of the animal. The pearl often has a NACREOUS appearance and although predominantly white in color, they may be various colors including blue, pink, green, brown, or nearly black. See also:
MOTHER OF PEARL.
See: Glossary of Pearl Terms by Pearl-Guide.com
See: Glossary of Technical Terms by Costellos.com