A pearl is not a mineral in the ordinary sense but an organic gem, built by a mollusc from layer on layer of nacre, microscopic platelets of aragonite, a form of calcium carbonate, cemented by the protein conchiolin, deposited around an irritant inside the shell. The interference of light in those thin layers produces the soft internal glow called orient.
Saltwater pearls from marine oysters were historically the most valued, and natural pearls, formed without human intervention, were among the costliest of all gems before the twentieth century, when Kokichi Mikimoto's perfecting of culturing around 1900 made bead-nucleated pearls widely available and collapsed the value of the natural stone. In Indian jyotisha the pearl is the primary gem of the Moon, Chandra, and one of the navaratna; across many cultures it has symbolised purity and the moon. Because pearl is soft, hardness about 2.5, and dissolves in acid, it is the most delicate of gems.