The Mother of pearl, also known Nacre, is a smooth crystalline substance produced by some mollusks as an inner shell layer that creates the iridescent visual effect. It also makes up the outer coating of pearls. Nacre is made of the same material that forms the inside of the shell. This combination of calcium carbonate and protein is both lighter and stronger than concrete. This resilient material is lightweight and transparent, allowing light to pass through its surface, creating a subtle glow on the surface. Not all mollusk shells have a nacre coating. The primary distinction between a shell that has a mother of pearl coating and a shell that does not is the iridescent quality. Mother of pearl has a very distinct multi-colored effect and a faint glow similar to other optically impressive moon-like gemstones.