LL Demos & Labs: Rock Profile: Obsidian Family
This specimen comes from Glass Mountain in Northern California. Obsidian is formed from rhyolitic lava which contains a lot of silica, the material used to make glass. In fact, if you should ever walk across an obsidian deposit, it will sound like you’re walking over a field of broken 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.