Samples of obsidian. Credit: U.S. Geological Survey.
Obsidian is a dense volcanic glass, usually rhyolite in composition, and typically gray or black in color; there is a well known occurrence of it at Yellowstone Park, Wyoming. Compared with window glass, obsidian is rich in iron and magnesium; tiny (less than 0.005 mm) crystals of iron oxide within the glass cause its dark color. Hardness 5.5, relative density 2.4.
Obsidian is often formed in rhyolite lava flows where the lava cools so fast that crystals do not have time to grow. Glass, unlike crystals, has no regular structure and therefore fractures in smooth conchoidal (curved) shapes. The intersections of these fractures can form edges sharper than the finest steel blades. For this reason, obsidian was used by many native cultures to make arrowheads and blades.
The colors in obsidian result from the oxidation state of the chemical elements within the tiny minerals that are finely dispersed in the glass. Black color results chiefly from magnetite, Fe304. If the obsidian is highly oxidized, then the glass may contain hematite, which provides a reddish hue. Variations in the oxidation state of the iron (Fe) varieties imparts a slight greenish hue. Some obsidian is banded, a consequence of oxidation on a flow surface being folded into the lava as it continues to move.