Smoky quartz is a type of clear quartz that is stained brown or black. The dark color occurs when free silicon within the stone is irradiated. This occurs naturally in the environment though it can be reproduced artificially in a laboratory. Lab-darkened quartz is often exceptionally dark and uniform in color. Smoky quartz is most commonly found in in Brazil, Madagascar, and the Southwestern USA. A yellowish to greyish form called Cairngorm is found in the Cairngorm Mountains of Scotland.