A gemstone is a mineral, rock (as in lapis lazuli) or petrified material that when cut and polished is collectible or can be used in jewellery. Others are organic, such as amber (fossilised tree resin) and jet (a form of coal). Some beautiful gemstones are too soft or too fragile to be used in jewelry, for example, single-crystal rhodochrosite, but are exhibited in museums and sought by collectors.
Some gemstones are manufactured to imitate other gemstones, for example cubic zirconia is a synthetic diamond substitute. The imitations copy the look and colour of the real stone but possess neither their chemical nor physical characteristics. However, synthetic gemstones are not necessarily imitation. For example, diamond, ruby, sapphire and emerald can be manufactured in labs, which possess identical chemical and physical characteristics as the genuine article. Artificial corundums including ruby and sapphire are very common and they cost only a fraction of the natural stones. Artificial diamonds are manufactured as industrial abrasives. However, artificial diamonds in gemstone-quality on the other hand are still too costly to manufacture.
A gemstone is prized especially for great beauty or perfection. Hence, appearance is almost the most important attribute of gemstones. Their beauty must also be able to stand the test of time; if a gemstone is scratched or crumbled, it loses its value instantly. 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 shape of the natural crystal. It is unsurprising that diamond is prized highly as a gemstone, since it is the hardest substance known and is able to reflect light with fire and sparkle.
Traditionally, gemstones were classified into precious stones and semi-precious stones. Only four types of gemstones were considered precious. They were:
Nowadays, all gems are considered "precious," although the four original "cardinal gems" are usually - but not always - the most expensive. There are over 130 species of minerals that have been cut into gems with 50 species in common use. These include:
Gems are classified into different groups, species and varieties. For example, ruby is the red variety of the species corundum which belongs to the hematite group. Emerald (green), aquamarine (blue), bixbite (red), goshenite (colorless), heliodor (yellow), and morganite (pink) are all varieties of the species beryl.
Gems have a certain refractive index, a certain dispersion, a certain specific gravity, a certain hardness, a certain cleavage, a certain fracture, a certain lustre. They may exhibit pleochroism of a certain sort, or double refraction to a certain degree and have an optic sign. They may have a certain luminescence and a distinctive absorption spectrum.
Certain material or flaws within a stone may be present as characteristic inclusions. And the gem may occur in certain locations, "occurrence." Gems from different locations may display different characteristics which may aid in identification.