The traditional classification in the West, which goes back to the ancient Greeks, begins with a distinction between precious and semi-precious; similar distinctions are made in other cultures. In modern usage the precious stones are diamond, ruby, sapphire, and emerald, with all other gemstones being semi-precious.[3] This distinction reflects the rarity of the respective stones in ancient times, as well as their quality: all are translucent with fine color in their purest forms, except for the colorless diamond, and very hard, with hardnesses of 8 to 10 on the Mohs scale. Other stones are classified by their color, translucency and hardness. The traditional distinction does not necessarily reflect modern values, for example, while garnets are relatively inexpensive, a green garnet called tsavorite can be far more valuable than a mid-quality emerald.[4] Another unscientific term for semi-precious gemstones used in art history and archaeology is hardstone. Use of the terms 'precious' and 'semi-precious' in a commercial context is, arguably, misleading in that it deceptively implies certain stones are intrinsically more valuable than others, which is not necessarily the case.
In modern times gemstones are identified by gemologists, who describe gems and their characteristics using technical terminology specific to the field of gemology. The first characteristic a gemologist uses to identify a gemstone is its chemical composition. For example, diamonds are made of carbon (C) and rubies of aluminium oxide (Al
2O
3). Next, many gems are crystals which are classified by their 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.
Gemstones 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. Other examples are the Emerald (green), aquamarine (blue), red beryl (red), goshenite (colorless), heliodor (yellow), and morganite (pink), which are all varieties of the mineral species beryl.
Gems are characterized in terms of 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.
Gemstones may also be classified in terms of their "water". This is a recognized grading of the gem's luster, transparency, or "brilliance".[5] Very transparent gems are considered "first water", while "second" or "third water" gems are those of a lesser transparency.[
There are a number of[11] laboratories which grade and provide reports on gemstones.
Gemological Institute of America (GIA), the main provider of education services and diamond grading reports. And also Colourstone reports.
International Gemological Institute (IGI), independent laboratory for grading and evaluation of diamonds, jewelry and colored stones.
Hoge Raad voor Diamant (HRD Antwerp), The Diamond High Council, Belgium is one of Europe's oldest laboratories. Its main stakeholder is the Antwerp World Diamond Centre.
American Gemological Society (AGS) is not as widely recognized nor as old as the GIA.
American Gem Trade Laboratory which is part of the American Gem Trade Association (AGTA), a trade organization of jewelers and dealers of colored stones.
American Gemological Laboratories (AGL), owned by Christopher P. Smith.
European Gemological Laboratory (EGL), founded in 1974 by Guy Margel in Belgium.
Gemmological Association of All Japan (GAAJ-ZENHOKYO), Zenhokyo, Japan, active in gemological research.
The Gem and Jewelry Institute of Thailand (Public Organization) or GIT, the Thailand's national institute for gemological research and gem testing, Bangkok
Gemmology Institute of Southern Africa, Africa's premium gem laboratory.
Asian Institute of Gemmological Sciences (AIGS), the oldest gemological institute in South East Asia, involved in gemological education and gem testing.
Swiss Gemmological Institute (SSEF), founded by Henry Hänni, focusing on colored gemstones and the identification of natural pearls.
Gübelin Gem Lab, the traditional Swiss lab founded by Eduard Gübelin.
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