Mother-of-Pearl

Mother-of-pearl is the shining, iridescent lining that some molluscs lay down on the inside of their shells, and the same material makes up pearls. No single formal name settles it: Wikipedia files it under nacre and calls it a composite of mineral and organic matter made by some bivalves, sea snails and cephalopods, while Cunningham describes it only as the interior of various sea molluscs, so no one species, mineral or chemical name covers it. It is built of thin hexagonal tablets of aragonite, a form of calcium carbonate, layered with sheets of elastic protein; the tablets are about as thick as a wavelength of light, which gives the play of colour. The mollusc keeps adding layers as long as it lives, smoothing the shell and sealing off irritants, which is how pearls form. Pearl oysters, freshwater mussels and abalone have been its main commercial sources, and its trade now falls under CITES. It is inlaid in buildings, jewellery and musical instruments and made into buttons and caviar spoons. Cunningham gives it to the Moon and Neptune and to Water, and names protection and wealth as its powers: it is laid on newborn babies to guard them, and wrapped with silver and a banknote as a charm for money.