What is Obsidian?
Natural glass of volcanic origin that is formed by the rapid cooling of viscous lava. Obsidian is extremely rich in silica (about 35 to 80 percent), is low in water (less than 1% water by weight), and has a chemical composition similar to rhyolite. Obsidian has a glassy lustre and is slightly harder than window glass. Though obsidian is typically jet-black in color, the presence of hematite produces red and brown varieties, and the inclusion of tiny gas bubbles may create a golden sheen. Other types with dark bands in gray, green, or yellow are also known. Different obsidians are composed of a variety of crystalline materials. Their abundant, closely spaced crystallites are so numerous that the glass is opaque except on thin edges. Obsidian in attractive and variegated colors is sometimes used as a semi-precious stone.
