Quartz is one of the most common and varied minerals on earth, and its abundant colors produce many gemstone types. Amethyst and Citrine are the most popular and valuable gem varieties of Quartz, but other forms also make important gemstones. Chalcedony describes any form of Quartz that is microcrystalline, in compact form without any visible crystals. Chalcedony also has several varieties used as gemstones, most notably Agate, Carnelian, Tiger’s Eye, and Chrysoprase.