Obsidian

Obsidian, or Volcanic Glass, said to have been named after its discoverer Obsidius, is a black, greenish-brown or grey glass, with a brilliant vitreous lustre, and breaking, like artificial glass, with a conchoidal fracture. It often exhibits streakiness or fluxion-structure, and is full of microliths, and may be spherulitic or vesicular. It is found in Iceland, the Lipari Islands, Mexico, and elsewhere. In composition it is acidic, corresponding closely with sanidine felspar (q.v.), and being, no doubt, formed by rapid cooling. The ancient Mexicans quarried it for arrow-heads and knives at the Cerro de las Navajas or "Hill of Knives." It is sometimes still cut and polished for ornamental purposes, as it was by the Greeks and Romans.