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Geosciences LibreTexts

7.11: Color Grading

  • Page ID
    3500
  • Color grading (colored gemstones)

    The color grading system of the GIA separates "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 pavilion).
    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 lighting. 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 speech, we are actually referring 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 comparison purposes, several software programs have been created to replace the old plastic color swatches the GIA used to sell.

    Color Code Name
      R red
      oR orangey red
      RO/OR red-orange or orange-red
      rO reddish orange
      O orange
      yO yellowish orange
      oY orangey 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)

    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

    Figure \(\PageIndex{1}\): Tone Scale


    Although from the tone scale 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 above, a pure blue hue is mixed in with various amounts of white (from 2 to 5) and various amounts 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 at least 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.

        Figure \(\PageIndex{2}\): Saturation scale for medium-dark toned hues    
        Saturation red.gif    
        1 2 3 4 5 6    
        Saturation blue.gif