Mother of Pearl is the iridescent coating on the inside of many mollusk shells.  Is is so named because when an irritant gets inside a shell, the shell protects itself by coating the irritant with the same material (nacre) of its lining that creates pearls ( therefore, this substance is the creator or mother, of pearls). Nacre is the same substance that is deposited around an object that becomes lodged in the mollusk - either naturally or inserted by humans - to become a pearl.

Care:  Use mild soaps and water on mother of pearl to clean it, and avoid storing it with jewelry which may scratch it, wearing away the luminous layers of nacre.