|Gem Color Saturation
The term color saturation is a widely used term in the gem trade. Unfortunately it's a widely misused term as well. So it's worth understanding what gemologists mean by it, and how it differs from related color terms such as tone and hue.
Hue is the basic concept of color and refers to the property of things that leads us to label them as "blue," "red," "green," "yellow," and so on. You don't need to study color theory to know that there are many different sorts of reds. These many reds differ with respect to hue, saturation and tone.
For example, some reds have a slightly orange hue, while others have a bluish hue. There are continuous variations from one hue to another, constrained by the natural order of colors from red to yellow to green to blue to violet. The order of colors is often illustrated on a color wheel.
Color saturation, sometimes known as chroma, is a measure of the purity or intensity of color hue. A highly saturated color will contain a very narrow set of wavelengths and appear much more pronounced than a similar, but less saturated color. Colors of low saturation are sometimes called weak, grayish or brownish while those of high saturation are said to be highly saturated, strong or vivid.
Tone represents the depth of color, ranging from colorless to black. In general, as light colors become darker they become more saturated. But as a color becomes very dark, verging on black, tone increases and saturation decreases. Gemstone tone is often described as "light," "medium-light," "medium," "medium-dark," and "dark."
The most valuable gemstones are those that exhibit a pure color and only slight hues of other colors in addition to their primary color. In practice, such purity of color is extremely rare. Almost all blue sapphires, for example, have a secondary hue, whether it be violet or green. Similarly, most emeralds have a secondary hue of blue or yellow.