Counterclockwise from top: obsidian, pumice and rhyolite (light color)
Obsidian is a natural volcanicglass: it is a kind of igneousrock. It is produced when lava high in silica (SiO2) cools rapidly, and solidifies quickly. Because there is no crystal structure, obsidian blade edges can reach almost molecular thinness. They were used as arrowheads, and today they may be used as surgical scalpel blades.