Of the 4Cs, emerald color may be the most important factor in grading emeralds. Emerald color in turn is a combination of hue, tone, intensity (saturation), and distribution. The finest emeralds display an intense green the color of young grass - a pure spectral green.

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

Hue
The hue of an emerald refers to the shade, tint or sensation of color. The most valuable and beautiful emeralds display "intense bluish" in addition to their basic green 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. A medium-dark green tone is considered best for emeralds.

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 emerald 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.