Citrine is quartz generally occurring in a golden color. Citrine occurs naturally in Scotland and in the Ural Mountains of Russia. Much citrine available today is actually smoky quartz or amethyst that has been heat-treated to achieve its characteristic color.
The relatively abundant mineral smoky quartz ranges in color from almost black to nearly colorless. Gem quality specimens are mined from the Swiss Alps and from Pike's Peak in Colorado. Dark smoky quartz is sometimes heat-treated to give it a lighter color which appears more brilliant when cut.