Pitchstone
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, however, unlike a glass pitchstone has an irregular, hackly fracture not a conchoidal fracture, this is due to its coarser (than obsidian) crystal structure. pitchstone has a resinous lustre, or silky in some cases, 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.