Obsidian is a type of naturally occuring
glass, produced from volcanoe
?s when the right kind of lava
? cools rapidly, e.g., by flowing into a body of
water. It consists mainly of SiO
2 (silicon dioxide). It's
mineral-like, but not a true mineral because it's not
crystalline. It's sometimes classified instead as a
mineraloid.
Its colour varies depending on the presence of impurities. It may contain patterns of air bubbles remaining from the lava flow.
It was highly valued in certain stone age cultures because it can be fractured to produce sharp blades or arrow heads. It may also have been polished to create early mirrors.