Pure obsidian is usually dark in appearance, though the color varies depending on the impurities present. Iron and other transition elements may give the obsidian a dark brown to black color. Most black obsidians contain nanoinclusions of magnetite, an iron oxide. Very few samples of obsidian are nearly colorless. In some stones, the inclusion of small, white, radially clustered crystals (spherulites) of the mineral cristobalite in the black glass produce a blotchy or snowflake pattern ("snowflake obsidian"). Obsidian may contain patterns of gas bubbles remaining from the lava flow, aligned along layers created as the molten rock was flowing before being cooled. These bubbles can produce interesting effects such as a golden sheen ("sheen obsidian"). An iridescent, rainbow-like sheen ("fire obsidian") is caused by inclusions of magnetite nanoparticles creating thin-film interference. Colorful, striped obsidian ("rainbow obsidian") from Mexico contains oriented nanorods of hedenbergite, which cause the rainbow striping effects by thin-film interference.