Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock. Obsidian is a popular gemstone. It is often cut into beads and cabochons or used to manufacture tumbled stones.
Pure obsidian is usually dark in appearance, though the color varies depending on the presence of impurities. Iron and magnesium typically give the obsidian a dark brown to black color.
Obsidian is sometimes faceted and polished into highly reflective beads. Some transparent specimens are faceted to produce interesting gems. It presents a different appearance depending on how it is cut;: in one direction it is jet black, while cut in another direction it is glistening gray.