Of the 4Cs, ruby color may be the most important factor. Ruby color in turn is a combination of hue, tone, intensity, and distribution. The color of ruby ranges from bright red to dark reddish-brown. The most preferred color for rubies is a deep blood red with a slightly bluish hue - such rubies are known as Pigeon's Blood Ruby or sometimes Burmese Ruby. Many rubies on the market are from Thailand, which have a brownish hue to them.

Rubies from Burma are famous for their exceptional red color. However, Burmese rubies rarely exceeds several carats and large flawless Burmese rubies can be worth millions of dollars.

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

Hue
The hue of an ruby 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.

Color zoning is present in certain rubies. Color zoning or distribution is visible as a darker band or bands across the gemstone.

Keep in mind that the color of an ruby 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.