Quartz gemstones are commonly separated into two groups based on the size of their individual crystals. The macrocrystalline quartz (large crystal) group includes many popular gemstones such as amethyst, ametrine and citrine. Cryptocrystalline quartz includes species whose individual crystals are too small to be easily distinguished. Apart from being a variety within the group, chalcedony is also a catchall term to describe cryptocrystalline quartz and includes many gems that have been coveted since antiquity.
