Gems are classified into different groups, species, and varieties. For example, ruby is the red variety of the species corundum, while any other color of corundum is considered sapphire. Emerald (green), aquamarine (blue), bixbite (red), goshenite (colorless), heliodor (yellow), and morganite (pink) are all varieties of the mineral species beryl.
A gemstone is prized especially for great beauty or perfection so appearance is almost always 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 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, 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.