Cultivation of pearls
As a reaction to an injury or a tiny foreign body that enters the mantel tissue, the oyster secretes mother of pearl, a crystalline substance, that coats many layers around the foreign body to form a pearl.
Of the over 8000 known species of double-shelled mussels, only about 20 are capable of producing pearls. The genus Pinctada produces the highest quality pearls, it can only be found in salt water. Pearl mussels of the genus Hyriopsis live in fresh water.