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GEMSTONES

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Gem Stones: 

Any of several gems, including the diamond, emerald, ruby, and sapphire, that have high economic value because of their rarity or appearance.

A gemstone is a mineral, rock, such in the case of lapis lazuli, or petrified material that when cut or faceted and polished is collectible or can be used in jewellery. Others are organic, such as amber, which is fossilised tree resin and jet, a form of coal. Some gemstones which may be generally considered precious or beautiful are too soft or too fragile to be used in jewelry, for example, single-crystal rhodochrosite, but are exhibited in museums and are sought by collectors. 

Characteristics and classification
Gemstones are described by gemologists using technical specifications. First, what is it 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 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, the "occurrence." 

Value

Jewellery made with gem amberA gemstone is prized especially for great beauty or perfection so appearance is almost the most important attribute of gemstones. 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 form of the natural crystal. 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 year. 

Traditionally, common gemstones were classified into precious stones (cardinal gems) and semi-precious stones. The former category was largely determined by a history of ecclesiastical, devotional or ceremonial use and rarity. Only five types of gemstones were considered precious: diamond, ruby, sapphire, emerald, and amethyst. In current usage by gemologists, all gems are considered precious, although four of the five original "cardinal gems" are usually—but not always—the most valuable. 

Rare or unusual gemstones, generally meant to include those gemstones which occur so infrequently in gem quality that they are scarcely known except to connoisseurs, include andalusite, axinite, cassiterite, clinohumite and iolite. 

Factors Influencing Esteem
Factors influencing the esteem in which gems are held are attractiveness, durability, rarity, fashion, and size. 

Synthetic and artificial gemstones
Some gemstones are manufactured to imitate other gemstones. For example, cubic zirconia is a synthetic diamond simulant composed of zirconium oxide. The imitations copy the look and colour of the real stone but possess neither their chemical nor physical characteristics. However, true synthetic gemstones are not necessarily imitation. For example, diamonds, ruby, sapphires and emeralds have been manufactured in labs, which possess very nearly identical chemical and physical characteristics to the genuine article. Synthetic corundums, including ruby and sapphire, are very common and they cost only a fraction of the natural stones. Smaller synthetic diamonds have been manufactured in large quantities as industrial abrasives for many years. Only recently, larger synthetic diamonds of gemstone quality, especially of the coloured variety, have been manufactured. 

Gemstone list
There are over 130 species of minerals that have been cut into gems with 50 species in common use. These include: 

Agate 
Alexandrite and other varieties of chrysoberyl 
Amethyst (originally a "cardinal gem", but now no longer so, since huge quantities were discovered in Brazil and the price plummeted) 
Aquamarine and other varieties of beryl 
Chrysocolla 
Chrysoprase 
Diamond 
Emerald 
Feldspar (moonstone) 
Garnet 
Hematite 
Jade - jadeite and nephrite 
Jasper 
Kunzite 
Lapis lazuli 
Malachite 
Obsidian 
Olivine (Peridot) 
Opal (Girasol) 
Pyrite 
Quartz and its varieties, such as tiger's-eye, citrine, agate, and amethyst 
Ruby 
Sapphire 
Spinel 
Sugilite 
Tanzanite and other varieties of zoisite 
Topaz 
Turquoise 
Tourmaline 
Zircon 

Minerals that infrequently occur in gem quality form: 

Andalusite 
Axinite 
Benitoite 
Bixbyte (Red beryl) 
Cassiterite 
Clinohumite 
Iolite 
Kornerupine 
Natural moissanite 
Zeolite (Thomsonite) 

Artificial or synthetic materials used as gems include: 

High-lead glass 
Synthetic cubic zirconia 
Synthetic corundum 
Synthetic spinel 
Synthetic moissanite 

There are a number of organic materials used as gems, including: 

Amber 
Bone 
Coral 
Ivory 
Jet (lignite) 
Mother of pearl 
Ammolite - from fossils formed from the shells of extinct ammonites. 
Pearl 
Tortoiseshell 

References
Weinstein, Michael, 1958, The World of Jewel Stones, Sheridan House, New York 
The Audubon Society Field Guide to North American Rocks and Minerals, 1978, New York, Alfred A. Knopf ISBN 0394502698 
Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., John Wiley and Sons, New York ISBN 0471805807 

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