Obsidian << ob SIHD ee uhn >> is a natural glass formed from lava flows that cooled quickly. Most obsidian has the same chemical composition as granite. But it cooled too quickly for the minerals to form crystals. The result is a smooth glass instead of granite. Most obsidian is black, or black with red bands. It is brittle. Native Americans used it to make tools, arrowheads, and knives. See also Granite; Igneous rock.
