Obsidian is a felsic (from feldspar + silica) rock that results from lava cooling so fast after eruption that the atoms do not have time to form crystal structures before the rock hardens. The dark rock has a very smooth, translucent surface, which is often described as volcanic glass. Due to the glass's lack of orderly crystal structure, it cannot be considered a mineral, but rather a mineraloid. Impurities such as iron and magnesium vary the color from dark green, to brown, to black, and sometimes white spots are observed. The color can also change depending on the direction the rock is cut. Like flint, obsidian is hard and fractures into sharp pieces; therefore, it once served well for cutting and piercing tools (arrows and spears; polished for mirrors) and is still used today for surgical purposes. As an alternative purpose, obsidian is thought by some to protect the sensitive against depression- "the stone of the soft-hearted and gentle people of the world."
Obsidian can be found all over the world near volcanic eruptions. It is common is the western United States (with an entire mountainside of obsidian located in Yellowstone National Park) as well as the state of Virginia.
http://crystal-cure.com/obsidian-gem.html http://en.wikipedia.org/wiki/Obsidian
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