Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock. It is produced when felsic lava extruded from a volcano cools rapidly without crystal growth. Pure obsidian is usually dark in appearance, though the colour varies depending on the presence of impurities. Iron and magnesium typically give the obsidian a dark green to brown to black colour. A very few samples are nearly colourless. In some stones, the inclusion of small, white, radially clustered crystals of cristobalite in the black glass produce a blotchy or snowflake pattern (snowflake obsidian). It 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) or a rainbow sheen (rainbow obsidian).