Obsidian, a volcanic glass, usually of rhyolitic composition, forms by rapid cooling of a viscous lava. Most obsidians are more than 70 percent silica and are low in volatile contents. Microscopic crystals of quartz or feldspar are sometimes included in the glass. Obsidian occurs as thick, short flows or domes over volcanic vents. It is usually black in color but occasionally red or brown (if iron-oxide dust is present), clear, or green. Obsidian displays a well developed conchoidal fracture, which makes it an excellent material for arrowheads, knives, and other sharp tools and weapons. Archaeologists use obsidian tools to trace trade routes, because such tools are relatively rare and each occurrence has a slightly different chemical composition. Thus the source of primitive obsidian tools may be located even if the tools have been traded across thousands of kilometers.

Obsidian has also been used as a semiprecious gem because of its shiny luster. Perlite, a hydrous form of obsidian, is used as a lightweight aggregate. When heated it expands into an artificial pumice like material. Its potential expansion after transport is a major shipping advantage. The rounded nodules of obsidian left after hydration and alteration of surrounding material into perlite are known as Apache tears. Looking like black teardrops, they are collector's items in the American Southwest. Now used in rock wool.