Natural citrine is a rare form of quartz crystal found in amethyst deposits. Many natural citrines may have started out as amethyst but heat from nearby magmatic forces caused the change to citrine. However, most citrines on the market today are heat treated amethyst or smokey quartz, cooked at high temperatures to produce the orange-yellow colors currently in demand.
Many citrine as well as amethyst mines are near the equator, including those in Brazil, currently the world's largest producer of heat-treated citrine (from amethyst). Due to operational costs, lack of electricity, or the expense and time to procure an electric or gas oven in Brazil's remote mining areas, the preferable method of heat-treating is often to bury the brownish (low grade) amethyst in the soil, cooking it with sunlight.
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Citrine is considered to be the most valuable quartz gem.