LL Demos & Labs: Rock Profile: Obsidian Family
This specimen comes from Northern California near Glass Butte. Obsidian is formed from rhyolitic lava which contains a lot of silica, the material used to make glass. Because rhyolite is a thick, sticky kind of lava, it oozed from the ground slowly. It also cooled quickly, which prevented large crystals from forming and gave it its glassy appearance. The black color is produced by the presence of iron. The thing that makes this obsidian different is that it hit a pocket of water and it was mixed in with the lava. When the lava hit the surface the it boiled producing tons of tiny bubbles which you see in the sample. This rock is used in the restaurant industry to clean grills.