Natural Pearls

A natural pearl forms when some foreign matter enters the body of a mollusk. The mollusk deposits layers of nacre over the intruder to soothe the irritation. 

Saltwater Cultured Pearl

Saltwater cultured pearls form when a bead nucleus and a piece of mantle tissue are inserted into the mollusk. This process is called bead nucleation. The mollusk first deposits a layer of conchiolin over the bead nucleus. It then deposits nacre, which is composed of platelets of aragonite.  

Freshwater Cultured Pearl

For freshwater cultured pearls, the culturing process starts when a piece of mantle tissue is inserted into the mussel. This is called tissue nucleation. Although a bead isn't inserted, the host mollusk reacts to the intruding tissue in the same manner as with bead nucleation. First it secretes a layer of conchiolin, then layers of nacre. Freshwater pearls are almost entirely nacre.