Obsidian has been used as a gemstone since antiquity. Imagine a fine piece of mahogany furniture that gleams like glass--that's mahogany obsidian. Obsidian is a naturally occurring volcanic glass that's created when lava rich in feldspar and quartz is cooled too rapidly for crystals to form. It's not considered to be a true mineral because its composition is too complex and it doesn't have a crystalline structure.
In addition to its decorative use, obsidian has been used to make smooth and sharp surgical blades and arrowheads due to its lack of cleavage and acute edges when broken. Stone Age people used obsidian for weapons and implements, Native Americans formed it into arrowheads and the Incans used it for weapons, mirrors and masks.
Obsidian is named after Obsuis, a Roman who discovered a similar stone.