What is "pitchstone"

Pitchstone \Pitch"stone`\, n. (Geol.) An igneous rock of semiglassy nature, having a luster like pitch.

n. (context geology English) a natural glass, rich in microscopic crystallites, formed by the rapid cooling of lava or magma

n. dark acid granitic glass

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.

The pitchstone ridge of An Sgùrr on the Isle of Eigg, Scotland, was possibly formed as a lava flow in a valley.

Pitchstone from the Isle of Arran was used as the raw material for making various items from the Mesolithic through the Neolithic to the Early Bronze Age. Mesolithic use appeals to have been limited to the Isle of Arran itself while in later periods the material or items made from it were transported around Britain.