A gemstone is prized particularly for great beauty or perfection so appearance is almost always the most significant attribute of gemstones. Characteristics that make a stone beautiful or attractive are colour, unusual optical phenomena within the stone, an exciting inclusion such as a fossil, rarity, and sometimes the form of the natural crystal. Diamond is prized very much as a gemstone since it is the hardest naturally occurring 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, general gemstones were classified into precious stones (cardinal gems) and semi-precious stones. The former class 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" (excluding the now-common amethyst) are typically—but not always—the most valuable.
Rare or unusual gemstones, usually meant to include those gemstones which occur so infrequently in gem quality that they are only just known except to connoisseurs, include andalusite, axinite, cassiterite, clinohumite and iolite.