pitchstone n : dark acid granitic glass
Pitchstone is a dull black glassy volcanic rock formed when lava cools swiftly. It is similar to but coarser than obsidian.
The ridge of An Sgurr on the Isle of Eigg was originally formed as a lava flow in a valley. Pitchstone is very resistant to erosion.
pitchstone in German: Pechstein
pitchstone in Spanish: Vidrio volcánico