For a faceted gem, the true hue, saturation and tone will be seen in the areas of internal luster. 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 determing 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.
SatutationImagine drops of blue ink being added to water.
Notice that the Hue is not changing. It is the same in each instance.
Only the saturation is changing.
Each time ink is added, the saturation increases until it reaches maximum saturation.
ToneImagine 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 are 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.
The image below shows degrees of Saturation + Tone
