Pearl

Hard object produced within a living shelled mollusc

A pearl is a hard, shiny object produced within the soft tissue (specifically the mantle) of a living shelled mollusk. Like the shell of a mollusk, a pearl is composed of calcium carbonate (mainly aragonite or a mixture of aragonite and calcite) in a crystalline form, which is deposited in concentric layers. More commercially valuable pearls are perfectly round and smooth, but many other shapes, known as baroque pearls, can occur.

From Wikipedia, under CC BY-SA. More on occurri.