Color

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Doos 12:27, 13 August 2006 (PDT)

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.

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 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
Tone scale for a blue hue


Saturation

Saturation scale for medium dark hues
Saturation red.gif
1 2 3 4 5 6
Saturation blue.gif
Blue-hsv.jpg


Causes of color

The causes of color can be divided into 4 different theories:

  1. The Crystal Field Theory
    • Transition metal compounds (Malachite, Almandine)
    • Transition metal impurities (Emerald, Citrine, Jade)
    • Color centers (Amethyst, Maxixe-beryl)
  2. The Molecular Orbital Theory
    • Charge transfer (Sapphire, Iolite)
    • Organic coloration (Amber, Coral)
  3. The Band Theory
    • Conductors (Copper, Silver, Iron)
    • Semi-conductors (Galena)
    • Doped semi-conductors (Diamond)
  4. The Physical Properties Theory
    • Dispersion (Fire in Diamond)
    • Scattering (Moonstone, Cat's eyes, Stars)
    • Interference (Iridescence, Opal)
    • Diffraction (Opal)