Mother-of-pearl is a wall of microscopic bricks thin enough to bend light
Nacre, the shimmering lining of oyster and abalone shells, is built like a wall of tiny hexagonal bricks of aragonite glued together with springy protein. Each brick is thin enough to interfere with visible light, which creates the colour, and the layered design makes the material much tougher than its brittle mineral alone.
Also called mother-of-pearl, nacre is a composite of mineral and organic matter that some molluscs lay down as the inner layer of their shells, and pearls are made of it too. It turns up in ancient lineages of bivalves, gastropods and cephalopods, although most mollusc shells are lined with a porcelain-like layer instead. Pearl oysters, freshwater pearl mussels and sea snails such as abalone and turban shells all carry it.
Its tablets of aragonite, a form of calcium carbonate, measure 10–20 micrometres across and about 0.5 micrometres thick, stacked in parallel sheets separated by elastic biopolymers such as chitin and lustrin. Because that thickness is close to the wavelength of visible light, reflections reinforce or cancel one another depending on the viewing angle, producing structural colour rather than pigment. The composite has a Young's modulus of 70 GPa. Its brickwork layout stops cracks running straight through: a fracture finds it easier to veer into the soft organic layer, and tiny mineral bridges between tablets add friction as they snap. Materials scientists study it as a template for tougher synthetics.
Water matters. Dried out, nacre loses much of its strength and behaves like brittle pure aragonite, because moisture keeps the protein matrix pliable. How it forms is still not fully understood. In the pen shell Pinna nobilis, particles of about 50–80 nm cluster into fibre-like lines before merging, and each crystal brick is thought to shoot up to the full layer height, then spread sideways until it meets its neighbours, giving the hexagonal packing. A 2021 Nature Physics study of Unio pictorum found spiral defects in the first layers that drew together and cancelled out, leaving later layers orderly.
The mantle tissue secretes nacre for as long as the animal lives, smoothing the shell interior and sealing off parasites and grit under successive coats, which yields blister pearls or loose ones; the process is called encystation. People have prized it for buttons, jewellery, laminated wall and floor tiles and instrument inlay, from accordions and oud sound holes to violin bow frogs and trumpet valve buttons. Its international trade is regulated by the endangered species convention signed by more than 170 countries.
Source: Nacre
