Bone

Bone is a composite of about 65% mineral and 35% organic by mass, and neither component would be much use alone.

The mineral is carbonated hydroxyapatite in crystals a few nanometres thick — very small, which matters, because a crystal that small contains no flaw large enough to start a crack. The organic part is mostly type I collagen, laid down first as a scaffold with the mineral nucleating on and within it.

The result is stiff like a ceramic and tough like a polymer, and it is the combination that is remarkable. Engineering materials generally trade one against the other: ceramics are stiff and shatter, polymers absorb energy and bend. Bone gets both, at a density of about two, and it repairs itself and remodels its internal structure in response to load — which is the property no manufactured material has at all.

The toughness comes from structure at every scale, which is what makes bone the standard example of hierarchical design.

At the nanometre scale, mineral platelets sit inside and along collagen fibrils, so load transfers between a stiff phase and a compliant one continuously rather than at an interface that can fail.

At the micrometre scale, fibrils are bundled into lamellae wound helically around the Haversian canals, and the winding angle rotates from layer to layer — plywood, essentially, and for the same reason.

At the millimetre scale, the cement lines between osteons are weak on purpose. A crack running through bone reaches one and turns to run along it rather than continuing across, which is precisely the deflection mechanism nacre uses and is why both materials outperform their ingredients by orders of magnitude.

Bone also loses this. The collagen cross-links progressively with age, the material becomes less able to deform before fracturing, and most of the increase in fracture risk in older people is that toughness loss rather than the loss of mineral density that gets measured.

As a worked material, bone is the ordinary counterpart of ivory: buttons, handles, combs, dice, needles, inlay. It is more porous, shows Haversian canals under a lens, and is what most 'ivory' in an ordinary house actually is.

Industrially it is processed rather than carved. Calcined at around 1,000 °C it becomes bone ash — essentially pure calcium phosphate — and bone ash is what makes bone china: typically half the body by weight, and the reason bone china is whiter, more translucent and considerably stronger than hard-paste porcelain fired at the same temperature.

Charred without air instead, it becomes bone char, historically the decolourising filter in cane sugar refining and still used for that in some plants, and the source of the pigment bone black.

Boiled, it gives gelatin and hide glue. And in medicine, processed bone mineral is used as a graft scaffold, because the apatite the body builds with is the apatite it will accept.