Nacre. A layered structure of organic material and calcium carbonate, secreted by the shell on its inner surface. Akoya, white-lipped, abalone—every pearl-producing mollusc builds this layer.

Each sheet is roughly 500 nanometres thick—close to the wavelength of visible light, which is precisely what causes the interference. The iridescence is not pigment. It is a product of structure. The colour is not applied; it is generated by the manner of stacking.

One of the earliest materials used by humans for ornament. Nacre jewellery has been recovered from Mesopotamian sites.