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Hue
Hue is the first impression of an object's basic color. Hue is described as red, orange, yellow, green, blue, violet, purple, or a combination of these colors.
The GIA system uses 31 hue names. To describe a gem's color, it combines basic hue names (red, orange, yellow, green, blue, violet, and purple) with any modifying hue names that might be necessary, All hues are listed on this chart.
| Hue |
Abbreviation |
| Purple |
P |
| reddish Purple |
rP |
| Red-Purple or Purple-Red |
RP/PR |
| strong purple Red |
stPr |
| slightly purple Red |
slPr |
| Red |
R |
| orangy Red |
OR |
| Red-Orange or Orange-Red |
RO/OR |
| reddish-Orange |
rO |
| Orange |
O |
| yellowish Orange |
yO |
| Orangy Yellow |
oY |
| Yellow |
Y |
| greenish Yellow |
gY |
| Yellow-Green or Green-Yellow |
YG/GY |
| Hue |
Abbreviation |
| strong yellowish Green |
styG |
| Yellowish Green |
yG |
| slightly yellowish Green |
slyG |
| Green |
G |
| very slightly bluish Green |
VslbG |
| bluish Green |
bG |
| very strong bluish Green |
vstbG |
| Green-Blue or Blue-Green |
GB/BG |
| very strongly greenish Blue |
vstB |
| greenish Blue |
| very slightly greenish Blue |
vslgB |
| Blue |
B |
| violetish Blue |
vB |
| Bluish Violet |
bV |
| violetish Puple |
vP |
Tone
Tone is the degree of darkness or lightness of a color. The system divides tone into 11 levels ranging from colorless (transparent) or white (opaque) - designated as 0 - through increasingly darker grays to back - designated as 10.
In practice, only levels 2 (very light) though 8 (very dark) apply to transparent colored stones. Tones that are any darker or any lighter are nearly impossible for the human eye to detect. Three standard tone levels are 3 (light), 5 (medium), and 7 (dark). With a little practice you can estimate the values between them.
Saturation
Saturation is a color's strength or intensity. It's assessed on a six-level scale (1 though 6). If the hue has any brownish or grayish components, saturation is 3 or lower. If brown or gray appear distinctly, saturation is 1 or 2. If you can only see the slightest hint of brown or gray, then saturation is around 3. If there's no trace of brown or gray, saturation is 4 or higher, saturation rating of 4 or higher makes an attractive colored stone, all other things being equal.
For most colored stones, valuable colors tend to fall within medium-light to medium-dark tone (4,5,6) and higher saturation (4,5,6).