What Does the GIA Colored Gem Grading Code Mean?
by International Gem Society
“Cut Morganite at the Natural History Museum,” the Natural History Museum, London, UK, by Paul Hudson. Licensed under CC By 2.0.
"Question
What do the abbreviations and numbers mean that appear in the International Gem Society (IGS) Gem Price Guide? For example, under the entry for beryl you have prices for “Fine Color Morganite: slpR 3/4.”
Answer
Those letters and numbers are a gem grading code for describing colors. It’s part of the Gemological Institute of America’s (GIA) grading system for colored gemstones. Jewelers and gemologists use it frequently.
Breaking Down Colors: Hue, Tone, and Saturation
This system breaks down gem color into three components:
* Hue reflects the basic colors we see. When most people discuss “color,” they’re actually
referring to hue.
* Tone refers to a gem’s relative lightness or darkness.
* Saturation refers to the hue’s intensity.
Hue Abbreviations
In the GIA system, the basic hues are red (R), orange (O), yellow (Y), green (G), blue (B), violet (V), and purple (P).
Note that brown, white, and black aren’t considered hues. Tone and saturation describe these.
For colors that combine hues, we express the primary hue last and the modifier first. So, greenish yellow is a hue with more yellow than green. Yellowish green is a hue with more green than yellow. We express even blends with a hyphen, like yellow-green.
When describing gem values in reports, gemologists use the abbreviations and capitalize the primary color. Lowercase is used for all other letters. So, “gY” means greenish yellow.
You’ll also see additional modifiers, such as “sl” for slightly and “st” for strongly.
Going back to the morganite example, “slpR” means slightly purplish red.
Tone Values ... "
There are many sub-links in the article that will further help your understanding if you're new to the terms, and as a side note, I must say, whoever took the last pic of the morganite ring did neither themselves, nor the ring any favors putting it on that godawful background. It makes the ring look funky, and the stone look muddied and unappealing, so I can't for the life of me figure out why they bothered using it in the article, since it detracts from the ring that's supposed to be featured.
I'd be ashamed to admit I took that shot if it was mine. 
https://www.gemsociety.org/article/colo ... e-meaning/
