Colored Gemstone Grading Basics
Gemstone color grading is broken into three quantifiable categories: intensity (saturation), hue (color), and tone (lightness/darkness). The GIA specifies and labels up to thirty one gemstone hues. The "hue" is the actual "color" of the material (blue, geen, red, etc.).
Gemstone Hue Chart
The GIA nomenclature also specifies six levels of saturation ranging from "grayish" (neutral grey) to "moderately strong" to "vivid." The term "intensity" is also used to describe a stone's saturation.
Gemstone Saturation Chart
To describe the darkness or lightness of a colored stone, the GIA system has nine levels of tone ranging from "very very light" to "very very dark." A numerical value is assigned to each label for use in a grading report.
Gemstone Tone Chart
With the exception of hue, each of these parameters by themselves are relatively meaningless and they must be used in conjunction to tell the full story of a gem's color quality. Taken together, these three parameters will provide a simple numeric code that can quickly and accurately quantify gemstone color.
Gemstone Grading Sapphire Chart
Using a Kashmir or Cornflower blue sapphire as an example, in order to receive a 10 quality rating, the stone would posses a "violetish/blue" hue, with a 6 or "medium dark" tone and 6 or "vivid" saturation.
Transparent colored stones are divided into three type classifications depending on crystal growth characteristics, and then graded within their type class, for clarity.
The G.I.A. grading system for colored stones lists clarity type classifications as Type I, Type II, and Type III. The classifications are defined as follows:
These are gems that grow extremely clean in nature and usually have no eye-visible inclusions. An example is Aquamarine.
These are gems that typically grow with some minor inclusions in nature and the inclusions may be eye-visible. An example is ruby.
These are gems that typically grow with many inclusions in nature and the inclusions are usually eye-visible. An example is emerald.
Colored stones are also graded for cut. Cut grading includes observance and qualification of brilliance, proportions, bulge, overall symmetry, and finish.
The weight of diamonds and colored stones is expressed in carats. One carat consists of 100 "points" and is equal to 0.200 grams.