Gemstone Grading – Saturation & Tone

Color Saturation in Precious Gems

Imagine drops of blue ink being added to clear water.

Notice that the Hue is not actually changing. It is the same in each instance.

What is changing is the color saturation. In other words, the water is getting more of the same color..

Each time ink is added, the saturation increases until it reaches maximum saturation. At 100% saturation, the water will have as much color as is possible

Looking for Gemstone Saturation

Before determining the saturation and tone of the primary hue we first need to know where to look.

For a faceted gem, the true hue, saturation and tone will be seen in the areas of internal lustre. Light entering the gem through the table or crown reflects off pavilion facets internally then returns to the eye.

Light entering the gem through a pavilion facet, reflects off the opposite pavilion facet and passes to the eye.

These areas of brilliance will seem to “change places” with complementary dark areas. Look for pinpoints of light or color or tiny areas of brilliant color.

Dark areas are often referred to as “extinction” and can be observed as the gem is “rocked” under a light source. Do not try to determining hue, saturation, and tone from the overall body color.

The areas of extinction result from light entering the gem through the table, crown, or pavilion and striking an internal facet at such an angle that the light does not return to the eye, resulting in dead or very dark areas.

Note: Extinction is often used in referring to large areas of a poorly cut gem that are permanently dark. While the characteristic is different, the physics of light is the same.

Tone in Precious Gems

Imagine the same drops of blue ink being added to highly saturated red water.

Notice that the Saturation is not changing.

It is the same in each instance.

Only the tone is growing darker with each drop of ink that is added.

However – while the concepts of saturation and tone can be illustrated separately, they tend to be inseparable in observation.

Saturation simply means more of a color. Tone is the lightness or darkness caused by the saturation of the primary hue and additional hue components from other impurities.

Saturation + Tone

It should be apparent from the matrix of circles that gemstones which show maximum saturation are very desirable. But much of the desirability of a gemstone will be determined by the tone.

If the tone is too dark, the gemstone will have less appeal.

Assigning a Composite Grade

First: Determine Saturation and Tone Grades

 

Saturation Grade grading saturation in gems
Tone Grade grading tone in gems
Note: The saturation and tone swatches should only be considered to be guidelines

Then: Assign a Composite Grade by Using the Matrix

Saturation Grade
Tone
Grade
Weak Medium Good Strong Vivid
Very Light 1 2 3
Light 2 4 6 8
Medium 2 5 7 9 10
Dark 2 4 6 8
Very Dark 1 2 3


You can see in the above tables, it is a very rare gemstone that has ideal saturation and tone. That gemstone could be worth a lot of money!

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