Pitchstone

Pitchstone, so called on account of its resinous lustre, is a glassy volcanic rock, agreeing in chemical composition with felsite (q.v.). It contains abundant microliths, and occasionally in cooling has developed that combination of rectangular and concentric shrinkage known as perlitic structure. Its average silica percentage is 70, and its specific gravity 23. It occurs in Arran, Eigg, etc.