Obsidian

a glassy stone of volcanic origin. It varies in color from black to brown and red, or gray. Some specimens are almost as transparent as glass. The precious stone known as cat's-eye is a variety of obsidian. There is a huge cliff of obsidian in the Yellowstone National Park. New Zealand and Hungary are noted for it. There are obsidian quarries in California. When struck with a hammer it chips by what is known as a conchoidal or shell fracture into spreading semicircular chips with sharp edges. The ancient Mexicans took advantage of this property to manufacture obsidian knives, arrow heads, and points for their spears.