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. This color is spread unevenly throughout the lava. If you hold your sample up to a bright light, you’ll see some dark areas and other areas that are almost clear. If your sample has white spheres through out the black rock, and is named snowflake obsidian. The white spheres are mineral impurities made of a mineral called feldspar tridimite.