Pearls are formed in shellfish- especially oysters and mussels. They are formed as a natural defense against an irritant, such as a piece of grit. Layers of aragonite called nacre are secreted around the irritant and then it gradually builds up to form the pearl. This depends upon the type of water and other ambient factors. The chemical composition of pearls is Calcium carbonate with conchiolin and water. Pearl is the precious stone for Moon. The semi precious substitute for this is Moon Stone and Opal. "