Color (or the lack of color) is one of the prime aspects what determines the beauty of a gemstone. For every person the quality of color is depended on his or her personal senses, making color grading a very subjective matter that would be very hard to communicate with the lack of a general system to compare and describe colors.
Luckely the GIA has developed such a system, based on the work of Albert Munsell done around the turn of the 20th century.
Color grading (colored gemstones)
The color grading system of the GIA seperates "color" into 3 components which will be discussed in full below.
A typical notation for a colored gemstone in this system will look like the following sample.
When we judge colored gemstones we judge them "face up" (table up) while balancing the stone between our fingers in the palm of our hand. This lets the light be reflected in and out of the stone through the crown. When we judge reflected light, it is termed "key color" opposed to transmitted light which is named "body color" (viewed through the pavillion).
Diamonds are judged table down against a white background, but the focus of this system is on colored gemstones.
A very important ingredient in judging color is proper lightning. By convention we use "northern skylight" when judging gemstones (or southern skylight when you live south of the equator).
When we use the term "color" in daily speak, we are actually refering to the "hue". Hue is the first impression we get when seeing color. This hue is modified by tone and saturation.
The GIA color grading system uses 31 different hues to describe the primary key color of a gemstone. These 31 hues are used to compare the color against the color of the gemstone.
For comparising purposes, several sofware programs have been created to replace the old plastic color swatches the GIA used to sell.
|RO/OR||red-orange or orange-red|
|YG/GY||yellow-green or green-yellow|
|styG||strongly yellowish green|
|slyG||slightly yellowish green|
|vslbG||very slightly bluish green|
|vstbG||very strongly bluish|
|GB/BG||green-blue or blue-green|
|vstgB||very strongly greenish blue|
|vslgB||very slightly greenish blue|
|RP/PR||red-purple or purple-red|
|stpR||strongly purplish red|
|slpR||slightly purplish red|
|*This was changed from bP (bluish purple)|
Tone is the lightness or darkness in a gemstone.
There are 11 degrees of tone in the GIA color grading system, but only 7 of them are actually used (2 through 8).
|0||colorless or white|
Although from the tonescale given above one could think of midrange tones (such as 5) to be gray, this is not the case. Instead one should think of it as white or black mixed in with the pure hue.
In the image on the right a pure blue hue is mixed in with various amounts of white (from 2 to 5) and various ammounts of black (5 to 8).
Gray is a modifier of saturation, not of tone.
Saturation is the brightness (or purity, intensity) of a hue. The purer the hue, the higher grade it will get on the saturation scale.
The saturation scale goes from 1 to 6. Lower grades (1 to 3) can have a gray or brown modifier, while in grades above 3 these modifiers are absent.
Cool colors, like green and blue, have a gray modifier in low saturation. The warm colors (red, orange and yellow) have a brown modifier. Any stone that doesn't have a gray or brown modifier will atleast be a 4 on the saturation scale.
Saturation scale with descriptions:
- grayish (brownish)
- slightly grayish (brownish)
- very slightly grayish (brownish)
- moderately strong
The descriptions are only used in verbal communications.
|Saturation scale for medium dark toned hues|