The internal, nacreous parts of a shell, commonly of calcium carbonate in the form of calcite or aragonite. It can be polished up to a beautiful irredescent hue and used as an inlay or veneer in jewellery, furniture, and other decorative objects. "Interference" caused by small surface grooves on the surface — minute striations on thin layers of chitin — results in an apparent bending of light. Thin film interference causes the colouration. The colours displayed are dependent on the thickness of the layer of chitin or its index of refraction.