Obsidian A dense volcanic glass, usually rhyolite in composition and typically black in color. Obsidian is formed where the lava cools so fast that crystals do not have time to grow.
Fractured obsidian can form edges sharper than the finest steel blades.
For this reason, it was used by many native cultures to make arrowheads and blades. Weathered obsidian may take on a brown colour on the surface.