Mother of pearl is an iridescent shell that is coated in nacre. A pearl is formed when a freshwater mollusk coats a particle with layers of nacre. Mother of pearl is worth a lot of money in the jewelry market because it is derived from living creatures.
Nacre is organic matter that is secreted from the freshwater mollusk and acts as a defense mechanism. It protects itself from shell fragments, parasites or implanted beads. Nacre is iridescent because it is made up of calcium carbonate. At many different angles, prismatic colors are reflected off of the shell. Freshwater, saltwater, South Sea and Tahitian are four types of pearls used in jewelry.