Obsidian

Obsidian is also called natural glass or volcanic glass. Obsidian is hard, glossy, and black, and is volcanic in origin. It is often chipped into arrowheads or larger chunks used as weapons, but the ornamental grade of stone is usually polished and smoothed. An inferior form of obsidian (decreased value) is called pitchstone, and is both duller and rougher than volcanic glass. Obsidian is also used in carved figures and figurines, including the magical obsidian steed. A typical specimen has a base value of 10gp.