Gems of The World
“By far the largest supplier of natural citrine is Rio Grande do Sol state in southern Brazil. Citrine mines in the US are located in North Carolina, Colorado, and California. The most valuable stones are the darkest, sometimes known as ‘Madeira citrine’ for their resemblance to the color of fortified wine. In nature, citrine’s color is due to tiny impurities of ferrous oxide,”
-Gems of the World by Cally Oldershaw