When lava meets water and cools too rapidly to crystallise, the result is this natural glass. Mineralogically, it is not a stone—classified as an amorphous solid.

The Aztecs fashioned obsidian into blades and mirrors. Fractured, it breaks conchoidally, producing an edge thinner than a steel scalpel. Some surgical blades still use obsidian today.

In Japan, obsidian has been in use since the Jōmon period, with Shirataki in Hokkaido and Kirigamine in Nagano as key sources. Trace the trade routes of the Stone Age, and they map onto the distribution of obsidian. One stone, outlining an ancient network.