Pitchstone is a dull black glassy volcanic rock formed when viscous lava or magma cools quickly. It is similar to but coarser than obsidian. It is a volcanic glass with a conchoidal fracture (like glass), a resinous lustre, and a variable composition. Its colour may be mottled, streaked, or uniform brown, red, green, gray, or black. It is an extrusive rock that is very resistant to erosion.
The pitchstone ridge of An Sgùrr on the Isle of Eigg, Scotland, was originally formed as a lava flow in a valley.