Pitchstone

Pitchstone is a dull black glassy volcanic rock formed when a felsic, viscous lava or magma cools quickly. It is similar to obsidian but is defined by the International Union of Geological Sciences as having a higher water content. 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.