concept

Conchoidal fracture

A break that leaves a smooth, continuously curved surface, rippled like the inside of a shell, and it is what happens in a material with no crystal cleavage planes for the crack to follow. With no preferred plane, the crack picks its own path through the amorphous solid and thins the resulting edge until it has almost no thickness at all. In volcanic glass such as obsidian that edge is only about 2-3 nm across, against 20-50 nm for surgical steel. This is why a stone can hold a keener edge than metal, and why it can be shaped by controlled percussion rather than grinding.