Obsidian is a naturally-occurring glass, which was formed from molten magma that cooled too quickly to solidify into a crystalline form. Although usually associated with a black coloration, it can also be somewhat reddish, brownish, orangish and/or greenish. However, brightly-colored specimens sold as "red", "green" and, especially, “blue" obsidian are likely to actually be man-made glasses from China, as such hues do not occur in nature.

Having been used as a weapon and gemstone since ancient times, obsidian’s name is described within Pliny’s Naturalis Historia as having been derived from that of a Roman explorer who had previously discovered a similar-looking material in modern-day Ethiopia. However, obsidian sub-varieties are usually defined by their visual attributes, with the white cristobalite inclusions seen within “snowflake obsidian” being the best-known of these. Other examples include the spectral iridescent effect displayed by "rainbow obsidian" and the reddish-brown regions contained within "mahogany obsidian".