Yin-Yang pendants on genuine leather lace from New Zealand.

Mother of pearl is part of the shell of mollusks. In many cultures the material is used in utensils or art objects. Almost all shells consist of a number of layers, each with its own function, of which the mother-of-pearl layer is one of them. The thickness of the different layers varies per mollusc group. In most groups the mother-of-pearl layer is one of the thinner layers that make up the shell, in some the layer is (almost) absent, there are also groups where the shell mainly consists of the mother-of-pearl layer. The mother-of-pearl layer is always on the inside of the shell and is secreted there (like the rest of the shell) by epithelial cells of the animal's mantle tissue. Apart from providing strength, its main function is to smooth the inside of the shell. A second function is to defend the animal against parasites and harmful waste. When the animal is attacked by a parasite or irritated by an external object that the animal cannot eject, a process is initiated in which the object is encased in successive layers of mother-of-pearl. This process can continue uninterrupted until the shellfish dies. Usually such an object is located between the animal and the shell. In those cases, the shell will only show a thickening at that location: the object is absorbed into the shell itself. Less often, the object is not in this location and a completely loose bead will develop. Mother of pearl is made up of hexagonal aragonite platelets (calcium carbonate (CaCO3)) that are 10 to 20 µm wide and 0.5 µm thick. They are placed in a continuous parallel, thin layer. The layers are separated by sheets of organic matrices composed of elastic biopolymers, such as chitin, lustrin and silky proteins. This mix makes the material strong and resilient, which is also due to the adhesion caused by the "brick structure" of the plates. This recurring pattern greatly increases the hardness, almost rivaling that of silicon. The iridescent appearance of mother of pearl is due to the fact that the thickness of the aragonite platelets is approximately 0.5 micrometers, which is comparable to the wavelength of visible light. This results in constructive and destructive interference of light in different wavelengths, causing different colors of light to be reflected at different angles at which the mother of pearl is viewed.