Pearls are products of bivalve mollusks (mainly oysters and mussels). They are built up of nacre (mother-of-pearl), which is mainly calcium carbonate in the form of aragonite crystals, and an organic horn substance known as conchiolin that binds the micro-crystals concentrically around an irritant. Although pearls are not especially hard, only about 2.5 to 4.5 on the Mohs scale, they are extraordinarily compact and very difficult to crush.
The typical pearly lustre is produced by the overlapping platelets of aragonite and film of conchiolin nearest to the pearl surface. The colour of pearl varies with the type of mollusk and the water, and is dependent on the colour of the upper conchiolin layer. Pearls may be natural (occurring without human intervention) or cultured; and they may occur in the ocean or in freshwater
Pearls are said to give the wearer a sense of calmness and centeredness, and to promote faith, loyalty, truth and purity. They can especially enhance personal integrity.