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Rectangular with Truncated Corners
Cab. Half Oval
Square with Truncated Corners
Emerald cut in a longish
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Maw Sit Sit
The Purity of Precious Metals
Gemstone Colour Varieties
Weight Measurement For Gemstone
The Nine Sacred Navaratnas
Gemstone Color Grading
Physical Properties of Gemstones
Wedding Anniversary Gemstones
The Myth of Gemstones
AIGS Master Color Chart
AIGS Hardness Table
Gemstone Color Grading
Any single color can be described relative to three elements:
Hue Position (“What color?”)
Saturation (“How intense?”)
Tone (“How dark?”)
A. HUE POSITION
There are seven basic hues within the color spectrum of white light:
Other intermediate hues are also possible, but sometimes one of these hues may be present in greater quantities than the other. This is termed the ‘dominant’ or ‘primary’ hue, with the other lesser color the ‘secondary’ hue. For example, the dominant hue of ruby is red and the secondary hue may be either orange or purple. Whereas, for blue sapphire, the dominant hue is obviously blue while the secondary hue may be green or violet.
When naming such colors, the dominant hue is always mentioned after a secondary hue that has taken the “-ish” suffix. For example, when red is dominant over orange the hue would be called “orangish-red”, but when orange is dominant over red it would instead be known as “reddish-orange”.
However, when both component hues are present in roughly equal quantities, we can place them in any order and the “-ish” suffix is dropped. Therefore, when equal quantities of red and orange appear to be present within a hue, we can call it “red-orange” or “orange-red”:
*** WE DO NOT ASSIGN QUALITY GRADES FOR A COLOR’S HUE POSITION, WE JUST NAME IT ***
It is impossible to say that green, for example, is more beautiful than red, as preferences for a particular hue are a personal choice. Thus, we do not pass judgment on the quality of the hue position during “grading”, but instead simply just name it.
Also known as chroma or intensity, saturation refers to the amount of color that is present.
In terms of its effect on value, saturation is the most important color grading factor for rubies, with higher-saturation stones almost always preferred. However, blue is a color that can become overly dark when present at very high levels of saturation. Therefore, for blue sapphires, we are looking for the hue’s saturation to be as high as possible while still keeping the ‘tone’ (see below) acceptably light.
It is sometimes difficult to visualize the concept of saturation. However, it can help to imagine a situation where we are slowly adding ink to a glass of water. The color will gradually become a deeper blue as the saturation increases proportionally with the amount of ink present, but the hue position will stay the same:
Adding ink to water to illustrate the concept of saturation
In general, adding more facets to a gemstone tends to deepen its color. A faceted ruby would have a maximum saturation if it were as red as it could possibly be and would have a minimum saturation if the color flashes were the lightest shade of pink possible without being colorless. Similarly, a blue sapphire would have a minimum saturation if the hue was so light that the stone was almost colorless. These low-saturation blue hues tend to appear as slightly grayish, as can be seen in the “weak” beaker of water pictured above.
Also known as “lightness”, tone refers to how light, or how dark, a color is. It is usually measured on a point scale, with “zero” being colorless - or white for opaque materials - and the scale’s maximum possible value representing black.
Light and dark tones compared
Another way of thinking of tone is as the percentage of light absorbed by a colored gemstone. Transparent gems that absorb no light are colorless, while those that absorb all light will be black.
As stated above, the highest saturations of some colors are fundamentally lighter, or fundamentally darker, than others. For example, the most intense reds or greens will always be darker in tone than the most intense yellows, but both will be lighter in tone than the most intense blues or violets.
To help further visualize the concept of tone, we can again consider the situation where ink is being added to a glass of water. However, this time we are going to take the process one step further. As before, when we initially add ink to the water, the saturation and tone of the blue color will increase. However, if we then continue to add more and more ink to the glass, we will eventually reach a stage where the tone - i.e. the amount of light being absorbed by the ink - continues to increase, but the resulting blackness begins to mask the intensity of the blue color. In other words, adding too much ink eventually darkens the color so much that the black overshadows the blue hue and results in a decrease in its observed saturation.
This situation is mirrored in blue sapphires from such localities as China, Australia and Nigeria, where the high iron content can lead to the blue hue being overly dark in tone.