Rainbow Obsidian
A small cabinet polished rainbow obsidian. The rainbow's probably due to interference between trapped microcrystals, though a 2001 paper claims thin-film interference (in Mexican rainbows). I've included it here because it's at least 50% silicon dioxide (the same composition as quartz) - the remainder is inseparable (it's amorphous) oxides. This was very difficult to photograph - please excuse the lighting glints and odd-colored shadows. This one may be from the Glass (Obsidian) Buttes, Lake County, Oregon.