Citrine is a variety of quartz. Citrine crystals can form together with amethyst or smoky quartz to form a bi-colored quartz called ametrine. Natural citrine is pale yellow to pale orange, while the heat-treated material is dark orange-brown to reddish-brown. All of the heat-treated material has a red tint, while natural citrine does not.
Citrine ranges in colors of yellow, yellow-brown, orange, dark orange-brown and reddish-brown. Darker colors are considered more valuable, including the medium golden orange and dark sherry-colors.
Most citrine comes from Brazil, but almost all of the Brazilian material is heat-treated amethyst. Natural citrine can also be found in Russia, France, and Madagascar.