It is a 7.0 on the MOHS scale. Almost all citrine on the market is heat-treated amethyst. Natural citrine is not common and occurs sparingly in many large quartz deposits. All heat-treated material has a red tint, whereas the natural specimens do not. In some amethyst deposits, the amethyst has been partially or fully changed to brown citrine by natural means of heating, thereby transforming it into citrine.
Most material comes from Brazil, but almost all of the Brazilian material is heat-treated amethyst.
Citrine can also be found in the Ural Mountains of Russia, in Dauphine, France, and in Madagascar. Although, often cut as a gemstone, citrine is actually somewhat rare in nature. Specimens of low grade, inexpensive amethyst or smoky quartz are often cooked at high temperatures to produce the more profitable orange yellow citrine. Citrines whose colors have been produced by artificial means tend to have much more of an orange or reddish caste than those found in nature, which are usually a pale yellow. Darker colors are more highly prized, including the medium golden orange and dark sherry-colors.