Diamonds - Brilliant Jewelers
D, E, F:

G, H:
Near-colorless. Only a trained gemologist can identify a trace of color.

Near-colorless. Only a trained gemologist can identify a trace of color.

Slight yellow color slightly visible to the untrained eye.

K, L, M:
Slight yellow color visible to the untrained eye.

N, O, P, Q, R:
More yellow color visible to the untrained eye.

S through Z:
Yellow color obvious even to the untrained eye.

Related to diamond color, fluorescence is a unique effect that causes a diamond to produce a slight blue glow when exposed to intense, direct ultraviolet light. Some people seek diamonds that produce this unique effect, while others definitely want to avoid it it's purely a matter of taste.
   Color Grading from D to Z:
The most important thing to know about color when it comes to diamonds is, in general, the less color a diamond has, the more valuable it is, all other factors being equal. Diamonds are found in nature in a wide range of colors, from completely colorless (the most desirable trait) to slightly yellow, to brown. So-called fancy color diamonds come in more intense colors, like yellow and blue, but these are not graded on the same scale.

The color grading system for diamonds uses the letters of the alphabet from D through Z, with D being the most colorless and therefore the rarest and most valuable, and Z having the least color within the normal range, and being the least valuable, all other factors being equal. A diamonds color is determined by looking at it under controlled lighting and comparing them to the Gemological Institute of Americas color scale, which is based on a set of diamonds of known color. Here is a diagram showing how a diamonds color is graded:

What Color Grade is Best?
For the purist, look for a colorless diamond with a grade of D-F and a fluorescence rating of none, faint, or slight. For an excellent value in a diamond with no noticeable color to the unaided eye, look for a near-colorless grade of G-I, and a fluorescence grade of medium or strong blue. Or, if you'd rather not compromise on color but would like to stay on budget, choose a diamond with a good cut and SI1SI2 clarity and consider going with a very strong fluorescence. It will still be beautiful to the unaided eye and you may prefer the unique effect of a strong fluorescence

What Happened to A, B, & C?
When gemologists began using this scale they reserved the A, B, and C grades for more colorless diamonds yet to be graded none have been found to date.