Pearl

A pearl is a hard, glistening object produced
within the soft tissue (specifically the mantle) of
a living shelled mollusk or another animal, such
as a conulariid. Just like the shell of a mollusk, a
pearl is composed of calcium carbonate (mainly
aragonite or a mixture of aragonite and calcite)
in minute crystalline form, which has been
deposited in concentric layers. Pink, blue,
champagne, green, and even purple saltwater
pearls can be encountered.