Nacre

Nacre is the iridescent lining that some molluscs deposit on the inside of their shells, and it is one of the most studied composites in existence for a reason that has nothing to do with how it looks.

By volume it is about 95% aragonite — calcium carbonate, the same mineral as chalk in a different crystal form — and about 5% organic material, a matrix of chitin and protein called conchiolin. Aragonite on its own is brittle and unremarkable. Nacre is roughly three thousand times tougher, from a composition that is almost entirely the brittle component.

That gap is the whole subject. Everything nacre does comes from arrangement rather than ingredients, which is the clearest natural demonstration there is that a material's structure and its composition are different questions.

The structure is called brick-and-mortar, and the description is literal. Aragonite platelets roughly 0.5 micrometres thick and five to fifteen across are stacked in layers, each layer offset from the last, with an organic film a few tens of nanometres thick between them.

A crack cannot run straight through that. It reaches a platelet boundary and must either break the tablet — which takes far more energy than propagating through a homogeneous solid — or turn and travel along the soft layer, which lengthens its path enormously. The organic film also stretches and holds the surfaces together behind the crack tip, a mechanism called crack bridging that keeps the two faces from separating cleanly.

The iridescence comes from the same geometry and is a bonus rather than a purpose. The platelet layers are spaced at roughly the wavelength of visible light, so light reflected from successive layers interferes, and the colour depends on the viewing angle. There is no pigment in nacre at all; grind it up and it is white.

This is why nacre is the reference structure for tough ceramics. Ice-templated alumina, layered composites and freeze-cast structures are all attempts to build the same arrangement in materials that will survive a furnace, and none of them has yet matched a shell.

Inlay, above everything else: furniture, musical instruments, firearm and knife handles, and the position markers on almost every guitar fingerboard.

Buttons were the industrial use and a very large one. Ocean pearl buttons cut from shell were the standard for good clothing until the 1940s, and the Mississippi river mussel industry that supplied them was substantial enough to deplete the rivers it worked.

The residue of that industry is now its most valuable product. The bead at the centre of nearly every cultured saltwater pearl is turned from American freshwater mussel shell, because that shell has the density and expansion behaviour an oyster will accept without rejecting the implant.