Natural glass of volcanic origin that is formed by the rapid cooling of viscous lava. It has a glassy lustre and is slightly harder than window glass. It is typically jet black, but the presence of hematite (iron oxide) produces red and brown varieties, and tiny gas bubbles may create a golden sheen. It is sometimes used as a semiprecious stone. Obsidian was used by American Indians and others for weapons, implements, tools, and ornaments, and by the ancient Aztecs and Greeks for mirrors.