Obsidian, a natural form of glass. It is formed when lava, brought to the surface of the earth by volcanoes, cools so rapidly that crystals have no time to form. Obsidian is hard and brittle. It is opaque, except at thin edges, and is usually black; some obsidian is red, brown, or gray. Obsidian usually contains quartz. In the United States, it is found in the Western states. The American Indians made arrowheads from obsidian.