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Characteristics and classification of gemstones

Gemstones are described by gemologists using technical specifications.
First, what it is made of, its chemical composition. Diamonds for example are made of carbon (C), rubies of aluminium oxide (Al2O3).
Next, many gems are crystals which are classified by crystal system such as cubic or trigonal or monoclinic.
Another term used is habit, the form the gem is usually found in, for example diamonds which have a cubic crystal system are often found as octahedrons.

Gems are classified into different groups, species and varieties.
For example, ruby is the red variety of the species corundum that belongs to the spinel or hematite group. Emerald (green), aquamarine (blue), bixbite (red), goshenite (colorless), heliodor (yellow), and morganite (pink) are all varieties of the mineral species beryl.

Gems have refractive index, dispersion, specific gravity, hardness, cleavage, fracture and luster. They may exhibit pleochroism or double refraction. They may have luminescence and a distinctive absorption spectrum.

Material or flaws within a stone may be present as inclusions. The gem may occur in certain locations, the "occurrence."