Quartz, silicon oxide (silica), is one of the most abundant and ubiquitous minerals in the earth's crust. It is the primary constituent of beach sand and occurs in many colors, all of which are used as gemstones. Amethyst, the most valuable, is the purple variety, and citrine is the yellow or orange variety. The stone pictured is from Uruguay, where a unique occurrence produces transparent quartz that is color zoned from purple to yellow-orange. Gems cut from the material around the plane of the color change can display both hues, and are termed 'ametrine'.