Citrine is a light yellow to yellowish-brown variety of QUARTZ. Natural citrine is colored by trace amounts of ferric hydroxide. Most of the citrine on the market is actually heat-treated amethyst or smoky quartz.
As in other varieties of quartz, citrine has a hardness of 7, a specific gravity of 2.65, and chemically, is SiO2. It has a vitreous luster and belongs to the hexagonal crystal system.
When amethyst is heated to approximately 500-600 degrees C, a yellowish-brown citrine color is produced. Not all amethyst responds in like fashion. Most of the Brazilian citrine on the market comes from the Irai region of Rio Grande do Sul.