Obsidian, ob SIHD ee uhn, is a natural glass formed when hot lava flows onto the surface of the earth and cools quickly. Obsidian has the same chemical composition as granite, but it is quenched to make glass rather than cooled slowly to form crystals. Most obsidian is black, or black with red bands. It is brittle. Indians used it to make arrowheads and knives. See also Granite; Igneous rock.