Every gemstone has its own color. A blue sapphire is not simply a blue sapphire. It may be a midnight blue, a deep darkish blue or a light royal blue. There are also pink, yellow, purple, black, orange sapphires and other shades.
A ruby is not merely red. It may be pigeon blood red, pinkish red or violet red. And emeralds are found in a variety of colors from grass green to soft green, olive green darkish green and more.
The color of a gemstone is determined by three separate factors: hue, tone and saturation.
Color shades

Hue - Dictionary Definition

  1. A gradation or variety of a color; tint: pale hues
  2. The property of light by which the color of an object is classified as red, blue, green or yellow in reference to the spectrum
  3. Color: allhues of the rainbow
  4. Form of appearance
The hue of a gemstone has an impact on its value. Some shades are more sought after than others, some are rarer, and some, with varying measures of brown or grey tinge are considered better value for money for those on a more limited budget.

Tone - Dictionary Definition

  1. A quality of color with reference to the degree of absorption or reflection of light; a tint or shade; value
  2. The distinctive quality in which colors differ from one another. In addition, these differences are indicated through tint, shade, and a slight modification of any given color, including hue: green with a yellowish tone.
We use tone to indicate the intensity of color. Tones can range from clear (without color) to black. When we add light, dark, darkish or other adjectives to our color descriptions, we are referring to the tone of the color being described.
For example, if you are looking for a royal blue sapphire you may find stones in our collection described as light royal blue, medium royal blue, or dark royal blue depending on their tone.

Saturation - Dictionary Definition

The degree of chroma or purity of a color; the degree of freedom from admixture with white.
A gemstone with excellent saturation reflects a pure color, without brown or grey hues. Our top gemstones are rated with either excellent or very good saturation for a vibrant, bright color.

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Definition courtesy of Random House Unabridged Dictionary, Random House, Inc. 2006.