Obsidian

Obsidian is lava that cooled too fast for any crystal to form. It has the composition of a rhyolite — mostly silica — and the structure of a glass, which means it has no grain at all and fractures with complete freedom in any direction.

An obsidian edge can be a single molecule thick, several times finer than the best steel scalpel, and blades are still made from it for eye surgery where the cleanness of the cut matters more than durability. It is also brittle enough that such a blade survives one procedure.

It forms at the surface where viscous, silica-rich magma is chilled — the margins of a rhyolite flow, the rind of a dome. The same magma cooled slowly would have been granite; cooled quickly but not quickly enough, rhyolite; quenched, glass.

Glass is not stable in geological time. Obsidian slowly takes up water and devitrifies into fine crystals, which is why almost none of it is older than about twenty million years, and why the world's obsidian is concentrated at young volcanic centres.

That instability is also a clock. A fresh surface hydrates at a rate that can be measured, so the thickness of the hydration rind on a flaked edge dates when the flake was struck.

Obsidian did for archaeology what few materials can: each volcanic source has a distinct trace-element signature, so a flake found anywhere can be matched to the specific volcano it came from. That has let researchers map prehistoric exchange networks in detail — Anatolian obsidian in the Levant nine thousand years ago, Melanesian obsidian moved across three thousand kilometres of open ocean.

In Mesoamerica it was the primary cutting material into the sixteenth century. The Aztec macuahuitl, a wooden sword edged with obsidian blades, was reported by Spanish soldiers to cut through a horse's neck — a claim usually recorded with the note that they had reason to exaggerate and that obsidian is genuinely that sharp.