Chalcedony Chalcedony is a cryptocrystalline form of silica, composed of very fine intergrowths of quartz and moganite. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monoclinic. Varieties of...
Carnelian Carnelian is a brownish-red mineral commonly used as a semi-precious gemstone. Similar to carnelian is sard, which is generally harder and darker. Both carnelian and sard are varieties of the silica mineral chalcedony colored by impurities of iron oxide....
