Glassy volcanic rock

• Dark acid granitic glass

WordNet Word definitions in WordNet
n. dark acid granitic glass

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

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Pitchstone \Pitch"stone`\, n. (Geol.) An igneous rock of semiglassy nature, having a luster like pitch.

Wikipedia Word definitions in Wikipedia
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, ...