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, lustre or other physical properties that have aesthetic value. Rarity is another characteristic that lends value to a gemstone.

==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 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, 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."

==Value==

A valuable (colored) gemstone is prized especially for its great beauty, rarity or aesthetics. Although color plays a very important role in determining the value of a gemstone, many other factors influence its price as well: market supply (think of the fluctuations of Tanzanite prices), rarity (Red Beryl), popularity of a stone, market mechanisms etc.

An example of a gemstone for which high prices are determined, not by its rarity, but stability of the market, and also marketing and consumer perception as well, are diamonds.

Diamond is prized highly as a gemstone since it is the hardest naturally occurring substance known and is able to reflect light with fire and sparkle when faceted. However, diamonds are far from rare with millions of carats mined each ye

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