Obsidian is a natural glass. It is formed from volcanic lava that cooled too quickly for significant crystallization to occur. Hence it is amorphous, with no cleavage; fracture is concordat. Obsidian is usually black, bur brown, grey, and, more rarely, red, blue, and green material is found. The color may be uniform, striped, or spotted. Some inclusions give obsidian a metallic sheen, while internal bubbles or crystals (called crystallites) produce a "snowflake" effect (hence snowflake obsidian) or an iridescence seen as Flashes of color.

Obsidian is found in areas where there is, or has been, volcanic activity: for example, Hawaii (USA), Japan, and Java. Other localities include Iceland, Hungary, and the Lipari Islands off Italy, the former USSR, Mexico, Ecuador, and Guatemala. Dark nodules found in Arizona and New Mexico (USA) is called "Apache tears".

Obsidian has been used since prehistoric times for making tools, weapons, masks, mirrors, and jewellery. The very sharp shards of the natural glass have been fashioned as blades, arrowheads, and daggers. Today most obsidian jewellery comes from North and Central America.