A gemstone is a mineral, rock (such in the case of lapis lazuli), or petrified material that when cut or faceted and polished is collectible or can be used in Jewellery. Others are organic (such as amber, which is fossilised tree resin, and jet, a form of coal).

Gems are classified into different groups, species, and varieties. For example, ruby is the red variety of the species corundum, while any other colour of corundum is considered sapphire. Emerald (green), aquamarine (blue), bixbite (red), goshenite (colourless), heliodor (yellow), and morganite (pink) are all varieties of the mineral species beryl.

There are over 130 species of minerals that have been cut into gems with 50 species in common use. These include: