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Colored Gemstone Grading & Grading Reports
Gemstone Color Grading Basics
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 Hue Chart
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.).
This hue color chart is only an approximation of the GIA color-naming scheme, and should not be used to "evaluate" gem color. In addition, the accuracy and/or reliability of any color chart that is displayed on a website is only as "accurate" as the color calibration of the computer monitor it is displayed on.
Gemstone Saturation 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.
The above example shows blue as the hue, but these same levels of color saturation could be applied to any of the hues in the uppermost chart.
Gemstone Tone Chart
To describe the darkness or lightness (similar to opacity or transparency) of a colored or colorless stone, the GIA system uses a 0–10 numerical scale with levels of tonal value ranging from "0" which represents totally clear (colorless or white) and "1" which is known as "very very light" or "extremely light," to "10" which is known as "very very dark" to "black."
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 perfect "10" quality rating, the stone would posses a "violetish/blue" hue, with a "6" or "medium dark" tone, and a "6" or "vivid" saturation.
Please Note: The only truly accurate color evaluation chart is one that was printed by a respected testing authority — such as the GIA GemSet® hue/tone/saturation chart or Gemewizard's GemePro® color communication system [2] — using their proprietary CMYK/RGB/Hexadecimal color formulas that match their specific nomenclature and color-naming schemes.
Colored Gemstone Certificates & Reports
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.
Some level of standardization in the accepted naming and typing of gem species, colors and gem treatments is essential to fostering consumer awareness. There are seven gemological testing labs that belong to the "Laboratory Manual Harmonization Committee" (LMHC), which seeks to provide uniform standards in gem nomenclature (see list below). The World Jewellery Confederation (CIBJO) also seeks to encourage both harmonization, and international cooperation in jewellery industry trade practices.
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.
GIA Color Description/Grading System
One well done, and widely used, system for color description is that developed and taught by GIA (Gemological Institute of America). Although not universal, it is familiar world-wide, and the basis for most formal gem description and evaluation in the US and Europe.
Since the wavelengths and light colors grade into one another in infinitesimal changes, there are an essentially infinite number of hues which could potentially be described. Most of these hues would be indistinguishable from each other to our eyes, so GIA has settled on a group of 31 which humans with normal color vision (and some training) can discriminate. The set of plastic gem models below is a representation of those 31. (It should be noted that GIA has taken some liberties with the traditional "Roy G. Biv" spectral colors, deleting indigo, and adding purple after violet. In the set below, then, you see: red, orange, yellow, green, blue, violet and purple. The intermediates are described by terms like slightly, moderately and strongly to indicate a spectral hue modified to various degrees by those on either side of it on the spectrum. It also recognizes hues which are exactly 50/50 mixes such as red-orange and blue-green.
The hue, then, consists of the key (or spectral) color plus adjectives describing the hue and strength of the secondary modifying color, if any. For example: slightly purplish red, symbolized by "sl p R" (sl for degree, lowercase p for the secondary hue (purple), and upper case R for the primary hue, (red). The description strongly yellowish green: "st y G" would be decoded using the same logic. Once you have been trained to see nuances of color, you recognize that pure spectral colors in gems are quite rare, and as a result, costly. For example: a hue of simply "B" would be pure spectral blue and if other factors of color and clarity were good, the piece would command a premium price.
If you are thinking that it looks like there are more gradations in the blues and greens in the display above, than in the other colors, you are right. Human vision has finer powers of color discrimination in that part of the spectrum, which this system takes into account.
Each of the 31 hues exists in a range of tones from almost colorless to almost black. GIA labels the tones as 0 - 10. {0 ( appears colorless), 1 (extremely light,) 2 (very light), 3 (light), 4 (medium light), 5 (medium), 6 (medium dark), 7 (dark), 8 (very dark), 9 (extremely dark), 10 (appears black).
The figure below represents the 2-8 part of that range, which is, in the great majority of cases, the range for marketable colored gems. For most species the most valuable tones are in the 5-6 range. The set below is shown without hue, and it takes practice and patience for the would-be colored gem grader to learn to superimpose hue onto these, and get a valid tone reading.
An additional complication comes from the fact that gem species differ in their inherent tone ranges. For example, let's compare an aquamarine and a pyrope garnet each of tone 6. Objectively, each is exactly the same, but that depth of color is about the deepest that will ever be found for aquamarine and the about lightest possible for any pyrope. One should not be surprised, then, to find the aqua dealer calling her stone "very dark" and the garnet seller raving about how beautifully light his stone is when they are both "6"'s.
Finally, it's time to examine the most subtle aspect of gem color, saturation: in a manner of speaking, this measure is the degree to which the other spectral colors "muddy up" the main hue. Think of a can of pure red paint and start adding in various amounts of all the other colors--> the more you add of the other spectral hues, the "browner" the red will get. Now do the same thing with a can of pure blue: the more you add the "greyer" the blue will get. In general, desaturating "warm colors" makes them look brownish while the same effect in "cool" colors looks more grey. Therefore, GIA's system of describing saturation makes a distinction between cool and warm hues.
Six degrees are recognized ranging from: 1 (brownish/greyish), 2 (slightly brownish/greyish), 3 (very slightly brownish/greyish), 4 (moderately strong), 5 (strong), 6 (vivid)
**In the figures below, you can get a better idea of the saturation effect by looking at the flat end of the plastic gem replica, rather than the "gem part".
When giving a gem's formal color description in words, then, the gem below might be said to be: medium dark, slightly greyish, blue-violet. It sounds more natural to put the tone, saturation and hue in that order. In a numeric description as required in offical gem grading documents, however: the order would be: hue, tone and saturation, thus: BV 6/2
One final point on the GIA color grading scheme: Two non-spectral colors are used (in addition to the officially sanctioned 31) and those arepink (pk) and brown (br). If one were to strictly follow the GIA system, all shades of pink are really lighter tones of red, and brown is simplydesaturated orange. It is rather a matter of bowing to tradition and convenience to recognize pink and brown as "colors" in their own right. You will see evidence of this practice in the color description below.
The grading for a gemstone's clarity and color is based on the GIA colored stone grading systems. The systems are applicable for "transparent" colored gems (not diamonds), and do not apply to opaque gemstones such as play-of-color opal, coral, chalcedony, onyx, lapis, pearls, coral, etc.
Within the three types, gem clarity is graded similar to diamonds, beginning at VVS and going down to I-3. There is no Flawless grade for colored gemstones.
Gemstone Color Grading
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