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Color is undoubtedly the most important criteria for evaluating and selecting gemstones. Because gemstones come in a rainbow of colors, color grading can be very complex. The three key criteria experts look at are hue, tone, and saturation. These attributes combine to create the colors we observe. Lighting is another important factor; gemstone color and brilliance can appear very different depending on the light source.
The hue of a gemstone is its base color. Hues can combine to produce additional hues, such as orangish-red or purplish-blue. The Gemological Institute of America (GIA) has developed a list of 31 hues for describing gemstones: 7 base hues and 24 combination hues.
Gemstones with a single pure hue tend to be the most highly valued.
Tone describes the lightness to darkness of a color. A pink gemstone, for instance, might be categorized as “light pink” or “medium pink”. The GIA tone scale has a total of 11 points, going from colorless to black.
Saturation is the intensity or purity of a color. With lower saturation, gemstone color becomes less pure and a gray or brown hue can be seen. The most desirable gemstones are those with strong saturation and even color throughout.
According to the GIA saturation scale, there are six degrees of saturation, ranging from grayish or brownish to vivid. As saturation decreases, gemstones with cool hues can appear grayish, while gemstones with warm hues can appear brownish.