Chalcedony is the most common form of cryptocrystalline quartz. The term 'cryptocrystalline' means that the crystals forming the mineral are so small they are not recognizable with the naked eye, nor under a normal microscope. When present as a cryptocrystalline aggregate in a fibrous form it becomes the gemstone known as chrysoprase.
The term Chalcedony is used in two specific cases: first, for a specific type of microcrystalline structure characterized by alternating fibrous layers of cryptocrystalline quartz; second, more broadly to describe a range of gemstones that fit this description including translucent varieties such as Agate, Carnelian, Chrysoprase, Jasper and Onyx, as well as some opaque specimens.