Characteristics and classification

Gemstones are described by gemologists using technical specifications. First, what is it made of, or its chemical composition. Diamonds for example are made of carbon (C) and rubies of aluminum 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 octahedron's.

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.

Gems have refractive index, dispersion, specific gravity, hardness, cleavage, fracture, and lustre. 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, called the "occurrence."

Gemstone

A gemstone, gem or also called precious or semi-precious stone is a highly attractive and valuable piece of mineral, which -- when cut and polished -- is used in jewelry or other adornments. However certain rocks, (such as lapis-lazuli) and organic materials (such as amber or jet) are strictly speaking not minerals, but are still applied in jewelry and adornments, and are therefore often considered a gemstone as well. Some minerals that are too soft to be generally applied in jewelry may still be considered a gemstone because of their remarkable color, luster or other physical properties that have aesthetic value. Rarity is another characteristic that lends value to a gemstone.