Obsidian is natural glass of volcanic origin that is formed by the rapid cooling lava. It has a glassy luster and is slightly harder than window glass. It is typically black, but the presence of minerals may create a golden sheen,silver sheen and rainbow colors. Obsidian was used by American Indians and others for weapons, implements, tools, and ornaments, and by the ancient Aztecs and Greeks for mirrors.
Safe to fire in metal clay 1200 degrees, slow ramp.