Of the 4Cs, gemstone color may be the most important factor in grading colored gemstones. Gemstone color in turn is a combination of hue, tone, intensity (saturation), and distribution. The finest gemstones display pure spectral colors.

The widely accepted system of judging gemstone color is the one used by Gemological Institute of America (GIA). This system breaks color into the following components:

Hue
The hue of an gemstone refers to the shade, tint or sensation of color. Within hue are varying tones and saturations.

Tone
Tone refers to the relative lightness or darkness of a hue, ranging from colorless to black.

Saturation
Saturation refers to the strength or purity of a hue. Hues with a minimum amount of brown or gray are described as vivid or strong, or intense.

Keep in mind that the color of an gemstone will appear differently depending on the kind of lighting it is viewed under. The most neutral light is daylight at midday. It adds the least amount of color and thus makes it easier to see the various nuances of color.