Obsidian is an igneous rock that forms when molten rock material cools so rapidly that atoms are unable to arrange themselves into a crystalline structure. The result is a volcanic glass with a smooth uniform texture that breaks with a conchoidal fracture. Obsidian is usually an extrusive rock – one that solidifies above Earth’s surface. However, it can form in a variety of cooling environments. Black is the most common color of obsidian. However, it can also be brown or green. Rarely, obsidian can be blue, red, orange or yellow. The colors are thought to be caused mainly by trace elements or inclusions. As a “glass”, obsidian is chemically unstable. With the passage of time, some obsidian begins to crystallize. This process does not happen at a uniform rate throughout the rock. Instead it begins at various locations within the rock. At these locations the crystallization process forms radial clusters of white or gray cristobalite crystals within the obsidian. When cut and polished these specimens are referred to as “snowflake obsidian”. Rarely, obsidian has an iridescent or metallic “sheen” caused by light reflecting from minute inclusions of mineral crystals, rock debris or gas. These colored specimens are known as “rainbow obsidian”, “golden obsidian” or “silver obsidian”, depending upon the color of the sheen or iridescence. These specimens are very desirable for the manufacture of jewelry.