When a mollusc or an oyster experiences an intrusion from a foreign body such as a piece of shell or a grain of sand, it covers itself in a layer of nacre. As more and more layers are added, a Pearl begins to form. Because the nacre is built up in layers, as light hits the surface of a Pearl, wonderful flashes of a multitude of colours can be seen. This iridescence occurs as the different wavelengths within light strike the layers at differing angles (see also iridescence). The inner part of a shell, the Mother of Pearl, is also a form of nacre. The better the quality of the nacre, the better its iridescence (also referred to as pearlescence).
In addition to Oysters, many other molluscs (a shell fish that has a hinge) have the ability to create nacre and therefore are able to produce Pearls.
