Comprehensive color grading is an extremely important factor in determining the value of colored gemstones. Unlike diamonds, where the cut is one of the most important factors, the "color" of colored gemstones such as emeralds, rubies, sapphires, and tanzanite can influence as much as fifty percent of a gemstone's value.
The evaluation of colored gemstones had no generally accepted system for quantifying color and color quality before the 1900's. An American painter and art instructor named Albert H. Munsell was the first to identify three basic qualities of a specific color - hue, saturation and tone. Albert Munsell authored two books on the subject: "A Color Notation" (1905) and the "Atlas of the Munsell Color System" (1915), and the GIA eventually adopted these principles to create a standardized gemstone color-grading system.
Gemstone Color Grading Basics
Gemstone color grading is broken into three quantifiable categories: intensity (saturation), hue (color), and tone (lightness/darkness). The GIA specifies and labels up to thirty one gemstone hues. The "hue" is the actual "color" of the material (blue, geen, red, etc.).
Gemstone Hue Chart
The GIA nomenclature also specifies six levels of saturation ranging from "grayish" (neutral grey) to "moderately strong" to "vivid." The term "intensity" is also used to describe a stone's saturation.
Gemstone Saturation Chart
To describe the darkness or lightness of a colored stone, the GIA system has nine levels of tone ranging from "very very light" to "very very dark." A numerical value is assigned to each label for use in a grading report.
Gemstone Tone Chart
With the exception of hue, each of these parameters by themselves are relatively meaningless and they must be used in conjunction to tell the full story of a gem's color quality. Taken together, these three parameters will provide a simple numeric code that can quickly and accurately quantify gemstone color.
Using a Kashmir or Cornflower blue sapphire as an example, in order to receive a 10 quality rating, the stone would posses a "violetish/blue" hue, with a 6 or "medium dark" tone and 6 or "vivid" saturation.
There are several trusted and reliable testing agencies that provide detailed gem grading reports but fall short when it comes to classifying colored gemstones. Reports for colored gemstones are not as systematic, methodical and detailed as a diamond grading report, but they do provide verified baseline data that can assist in appraising, isuring, and reselling a colored gemstone. Additionally, these reports vary significantly from agency to agency so if you are planning on obtaining a color grading report it is important to understand the difference.
GIA Identification Report
A GIA (Gemological Institute of America) report (above, center) provides detailed information about the dimensions, carat weight, shape and cut of a stone. GIA reports also cover any treatments or enhancements that have been done and of corse, wether the stone is natural or synthetic. A GIA report utilizes a "colored stone grading system" chart to graph the hue, tone, and saturation of a stone.
AGTA Identification Report
An AGTA (American Gem Trade Association) report (above, left) provides the usual detailed information about the dimensions, carat weight, shape, cut, and enhancements of a stone, and will also will include a photograph. The AGTA report will also comment on the lack of any heat-treatments and the corresponding increase in value for untreated stones. an AGTA report will also state the geographical origin of a stone (ie. ruby or sapphire), which can significantly affect its value (ie. Burmese ruby). AGTA does not however, use the GIAs more extensive hue scale and does not take into account the tone or saturation.
AGL Colored Stone Certificate
An AGL (American Gemological Laboratories') report (above, right) also provides detailed information about the dimensions, carat weight, shape, cut, and enhancements of a stone. AGL reports also include an illustrated diagram of the stone that shows cut and clarity information. AGL uses proprietary color and clarity grading that is completely different from the GIA standard and therefor, difficult for a layperson to understand. Color grading is defined as: "Color Rating/Tone" and is graded on a scale from 1 (excellent) to 10 (poor). A numeric value for color is given under "Color Scan". The testing light source is also identified. An AGL certification also provides information on the stone's country of origin, and identifies any gem enhancements or treatments.
GGL Gübelin Gem Laboratory Ltd.
The Gübelin Gem Laboratory in Lucerne, Switzerland is one of the leading testing laboratories in Europe, testing colored stones, fancy colored diamonds, colorless diamonds, and pearls. They are also on the cutting edge for testing diamonds for any sign of (HTHP) enhancement. Colored stones are tested for the usual parameters of clarity, color, cut, and enhancements as well as place of origin. Diamonds are tested for clarity, color, flourecence, cut, and symmetry. GGLs pearl report is fairly basic, and does not address nacre thickness.
Colored Gem Testing Laboratories
Gemological Institute of America (GIA)
5345 Armada Drive
Carlsbad, California 92008
Tel: (800) 421 7250
www.gia.edu
American Gem Trade Association (AGTA)
18 East 48th Street, Suite 502
New York, New York 10017 USA
Tel: (212) 752 1717
www.agta.org
American Gem Society Laboratories (AGSL)
8917 West Sahara Avenue
Las Vegas, Nevada 89117
Tel: (702) 233 6120
www.agslab.com
American Gemological Laboratories (AGL)
580 Fifth Avenue
Suite 706
New York, New York 10038
Tel: (212) 704 0727
European Gemological Laboratory USA (EGL)
6 West 48th Street
New York, New York 10036
Tel: (212) 730 7380
www.eglusa.com
Gübelin Gem Laboratory Ltd. (GGL)
Maihofstrasse 102
6006 Lucerne Switzerland
Tel: (+ 41) 41 / 429 17 17
www.gubelinlab.com
Swiss Gemmological Institute (SSEF)
Falknerstrasse 9
CH-4001 Basel, Switzerland
Tel: (+ 41) 61 262 06 40
www.ssef.ch
Österreichische Gemmologische Gesellschaft (SSEF)
Goldschlagstrasse 10
1150 Wien (Vienna), Austria
Tel: (+ 43) 01 231 22 38 od. 0676/310 40 66
www.oegemg.at
Laboratoire Français de Gemmologie (LFG)
2, place de la Bourse
75002 Paris, France
Tel: (+ 33) 1 40 26 25 45
www.diamants.ccip.fr
Gemmological Institute of India (GII)
29 Gurukul Chambers
187-189 Mumbadevi Road
Bombay 400 002, India
Tel: (+ 91) 22 2342 00 39
www.giionline.com
Gem Institute of Thailand (GIT)
Gem Research and Testing Building
Phayathai Road
Bangkok 10330, Thailand
Tel: (+ 66) 2 218 54 70-3
www.git.or.th
Gem Testing, Standards, & Trade Organizations
The World Jewellery Confederation (CIBJO)
(Confédération Internationale de la Bijouterie, Joaillerie et Orfèvrerie)
Piazzale Carlo Magno, 1
20149 Milano, Italy
Tel: (+ 39) 02 4997 7098
www.cibjonews.com
Bibliography & Suggestions for Further Study on Colored Gemstone Certificates
1. JCRS, Colored Stone Certificates 
. www.jcrs.com
2. Gemstone Forecaster, Grading Color - GIA's Colored Stone Grading System . www.preciousgemstones.com
3. Modern Jeweler, Guide to Gem Lab Reports . archives.modernjeweler.com
