A gemstone (gem) is a precious or semi-precious stone in cut and polished form. It is most often a piece of mineral crystal and is used to make jewelry or other ornamentations.
Although most gemstones are hard, there are some soft minerals that can be used in jewelry as well because of their sheen or other physical characteristics which are visually appealing.
Today, the stones considered to be precious are diamonds, emeralds, rubies, and sapphires. All other gemstones are considered semi-precious.
Gemologists identify Gemstones by characteristics such as Chemical Composition, Crystal System, etc.
Example: Diamonds are made of carbon (C)
Rubies are made of aluminum oxide (Al2O3), etc. etc.
Example: cubic, trigonal or monoclinic
The form the gem usually takes - Diamonds, have a cubic crystal system, and are often found as octahedrons, although they can also be cubic in form.
Example, Ruby is the red variety of the species Corundum, while any other color of Corundum is considered a Sapphire.
Example: Emerald (green), aquamarine (blue), red beryl (red), heliodor (yellow), and morganite (pink), which are all varieties of the mineral species Beryl.
Gems are also described in terms of 1) refractive index, 2) dispersion, 3) specific gravity, 4) hardness, 5) cleavage, 6) luster, etc.
Gemstones may also be classified in terms of their "water". This is in regards to the gemstone’s luster, transparency and/or "brilliance." As an example, gems that are very transparent are considered "first water", while gems considered "second" or "third water" are of a lesser transparency.