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For a colored stone (any gem other than diamond), color is the most important factor in determining quality.
To the color scientist, given an opaque, matt-finished object, there are three dimensions to color:
- Hue positions
- Saturation (intensity)
- Tone (lightness or darkness)
For colored gemstones, there is also a fourth factor:
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Left: Three-dimension view of a color solid. Illustration courtesy of Minolta USA.
Right: Hue position describes the position of a color on a color wheel. Illustration © R.W. Hughes |
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| The relationship between tone (lightness) and saturation.
Illustration © R.W. Hughes |
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Hue position
The position of a color on a color wheel, i.e., red, orange, yellow, green, blue and violet. Purple is intermediate between red and violet. White and black are totally lacking in hue, and thus achromatic (‘without color’). Brown is not a hue in itself, but covers a range of hues of low saturation (and often high darkness). Classic browns fall in the yellow to orange hues.
Generally speaking, gems with hues that most closely resemble the red, green and blue (RGB) sensors in our eyes are most popular. Thus the colored gem trinity, ruby, emerald and sapphire. But there is much about hue that is a personal preference and will depend upon an individual’s personal taste. |
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| Three green gems, showing a variation in hue position. The round center stone is a straight green, while the trillion-cut stone at left is a more yellowish green and the oval stone at right a slightly bluish green. Generally speaking, hue position is of less importance than saturation. Photo: Wimon Manorotkul |
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Satuartion (intensity)
The richness of a color, or the degree to which a color varies from achromaticity (white and black are the two achromatic colors, each totally lacking in hue). When dealing with gems of the same basic hue position (i.e., rubies, which are all basically red in hue), differences in color quality are mainly related to differences in saturation, because humans tend to be more attracted to highly saturate colors. The strong red fluorescence of most rubies (the exception being those from the Thai/Cambodian border region) is an added boost to saturation, supercharging it past other gems that lack the effect. |
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| Four blue sapphires showing a variation in saturation and tone. Stone 1 possesses a light tone and low saturation. Stone 2 is close to ideal in both tone and saturation. Stone 3 has greater saturation than Stone 2 in some areas, but its overall tone is too dark and it shows too much extinction. Stone 4 is so dark in tone that its saturation is reduced. Note that inclusions are far more visible in stones of light tone than those of dark tones. Photo: Wimon Manorotkul |
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Tone
The degree of lightness or darkness of a color, as a function of the amount of light absorbed. White would have 0% darkness and black 100%. At their maximum saturation, some colors are naturally darker than others. For example, a rich violet is darker than even the most highly saturated yellow, while the highest saturations of red and green tend to be of similar darkness. Note that as saturation increases, so too does tone (since more light is being absorbed. However, there reaches a point where increases in tone may result in a decrease in saturation, as a color “blackens.”
When judging the quality of a colored gem, tone is an important consideration. Before buying, it’s always a good idea to consider the lighting conditions under which it will be worn. Look for stones that look good even under the low lighting conditions you find in the evening or in a restaurant, for these are typically the conditions under which fine gems are worn and viewed. Also view gems at arm’s length and look for those that are attractive even at a distance. Exceptional gems will look great under all lighting conditions and viewing distances. |
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