Pearl is a beautiful natural response to irritation. Pearls are created when an oyster or other shellfish gets an irritant in its shell and responds with layering of soothing nacre on the object. Nacre is made of aragonite or a mix of aragonite and calcite. 

There are many different types of pearls that occur in both freshwater and salt water. Any more large pearls that are not cultured, or carved from another shell and put within an oyster to be layered with nacre, are very rare.