Obsidian
When lava erupts rapidly onto the Earth's surface, it is suddenly in an environment a thousand degrees cooler than the one it was in underground.
•Such a rapid temperature change causes the lava to freeze and solidify rapidly. The rock formed may have a glassy surface with no crystals visible.
•Obsidian is a rock that is formed under these conditions, and it contains no real crystals.
•With the help of microscopes and chemical analysis, geologists have found that obsidian is related to granite in composition and to another volcanic rock called rhyolite.
•Pitchstone is a rock that is rather like obsidian, but contains more crystalline material and its surface is more like pitch, or tar.
•When broken, obsidian has a very sharp, curved surface. This is called a conchoidal (shell like) fracture.
•Its sharp fracture has been exploited by primitive people for making weapons and other implements, as it can be easily and accurately shaped.
•Snowflake obsidian is an attractive rock covered with small white patches. These occur where the volcanic glass has deteriorated. This rock is often cut and polished.
•Obsidian is a well-known rock, but is not very common. Hecla in Iceland and Obsidian Cliff in Yellowstone National Park, are famous locations.