Obsidian

Is glass from a volcano!

It is produced when lava from a volcano cools rapidly with little crystal growth. Obsidian is commonly found at the edges oflava flows where the chemical composition (high silica content) makes thick gooey lava. Because it lacks crystal structure, obsidian edges can be very thin and sharp leading to its ancient use as arrowheads and knife blades, and its modern use as surgical scalpel blades.

 

For more information try these links:

http://stoneplus.cst.cmich.edu/obsidian.htm

http://volcanoes.usgs.gov/images/pglossary/obsidian.php

http://volcano.oregonstate.edu/education/facts/obsidian.html

http://geology.utah.gov/utahgeo/rockmineral/collecting/obsidn.htm