Color grading

From The Gemology Project
Revision as of 15:43, 18 March 2012 by Barbra (talk | contribs)
(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to: navigation, search

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.

  • Hue
  • Tone
  • Saturation

A typical notation for a colored gemstone in this system will look like the following sample.

B 5/2
The B stands for the hue (blue in this case),
5 stands for the tone and
2 indicates the level of saturation

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).

Hue

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.

Color Code Name
R red
oR orangy red
RO/OR red-orange or orange-red
rO reddish orange
O orange
yO yellowish orange
oY orangy yellow
Y yellow
gY greenish yellow
YG/GY yellow-green or green-yellow
styG strongly yellowish green
yG yellowish green
slyG slightly yellowish green
G green
vslbG very slightly bluish green
bG bluish green
vstbG very strongly bluish
GB/BG green-blue or blue-green
vstgB very strongly greenish blue
vslgB very slightly greenish blue
B blue
vB violetish blue
bV bluish violet
V violet
vP* violetish purple*
P purple
rP reddish purple
RP/PR red-purple or purple-red
stpR strongly purplish red
slpR slightly purplish red
R red
Pk pink (exception)
Brn brown (exception)
*This was changed from bP (bluish purple)

Tone

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).

Tone Scale Name
0 colorless or white
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 black
Hsv-blue.jpg


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

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:

  1. grayish (brownish)
  2. slightly grayish (brownish)
  3. very slightly grayish (brownish)
  4. moderately strong
  5. strong
  6. vivid

The descriptions are only used in verbal communications.

Saturation scale for medium dark toned hues
Saturation red.gif
1 2 3 4 5 6
Saturation blue.gif

Sources



Next: Causes of Color

Return to the Table of Contents