Pearl: A pearl is a generally spherical structure made up of calcium carbonate in crystalline form (aragonite) deposited in concentric layers, produced by living tissues - in particular by the mantle - of molluscs (typically oysters), bivalves and gastropods. Conularia also produced pearls, composed, however, of calcium phosphate. The term "pearl" derives from the Latin "pernula", the name used to indicate the shell that contains it, and whose shape recalls the "pig's leg".