Obsidian is dense volcanic glass usually black in color. Inclusions and
bubbles add color and patterns to some types of obsidian. It is often formed
in rhyolite lava flows when the lava cools quickly enough to prevent crystal
formation. Glass has no regular structure and fractures in round smooth
sections. These can form an extremely sharp edge and obsidian was used by
many cultures for knives and arrowheads. Today obsidian bladesare used
in heart surgery, and have a cutting edge much sharper than surgical steel
scalpels.
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