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.