Citrine is yellow to brown quartz with ferric impurities and rarely found naturally. It varies from pale yellow to rich golden yellow to dark orange. Although citrine may occur naturally, much is produced by heating amethyst or smoky quartz under controlled conditions. Darker colors are more highly prized, including medium golden orange ("Rio Grande" citrine) and dark sherry-colored ("Madeira" citrine). Brazil is the leading producer of citrine. It is nearly impossible to tell cut citrine from yellow topaz visibly.