Mother of pearl is an iridescent layer of material which forms the shell lining of many mollusks. Pearl oysters and abalone are both sources of mother of pearl. Mollusks create mother of pearl to protect themselves. In addition to forming part of the shell, mother of pearl also insulates mollusks from bacterial infection, and reduces irritation from organic material which drifts into the shell. Irregularities often form in the mother of pearl as a result of irritation. If the irritation is extensive enough, the mollusk will form a large bump, prized in the human world as a pearl.
Avoid storing mother of pearl jewelry with other jewelry that could scratch it, wearing away its luminous nacre.