A gemstone is prized especially for great beauty or perfection so appearance is almost the most important attribute of gemstones. Characteristics that make a stone beautiful or desirable are colour, unusual optical phenomena within the stone, an interesting inclusion such as a fossil, rarity, and sometimes the form of the natural crystal.
Diamond is prized highly as a gemstone since it is the hardest substance known and is able to reflect light with fire and sparkle when faceted. However, diamonds are far from rare with millions of carats mined each year.
Traditionally, common gemstones were classified into precious stones (cardinal gems) and semi-precious stones. The former category was largely determined by a history of ecclesiastical, devotional or ceremonial use and rarity.
Only five types of gemstones were considered precious: diamond, ruby, sapphire, emerald, and amethyst.
In current usage by gemologists, all gems are considered precious, although four of the five original "cardinal gems" are usually—but not always—the most valuable.
Rare or unusual gemstones, generally meant to include those gemstones which occur so infrequently in gem quality that they are scarcely known except to connoisseurs, include andalusite, axinite, cassiterite, clinohumite and iolite.