Diamonds with an absence of body color most purely reflect the full color spectrum and are therefore the most valuable. Most diamonds come in a wide range of colors and are graded on a letter scale from D (colorless), the highest grade, through Z (usually a light yellow, brown, or gray). It is difficult for the untrained eye to see variations in color grades D through J unless stones are being compared side by side.
Diamonds should be color-graded under exacting conditions by an expert grader using specially filtered cool white light. They should be compared to a set of diamonds with known colors, graded by the Gemological Institute of America against their master set. To obtain the most accurate color grade, graders usually place diamonds on their sides or upside-down against a neutral background, to help reduce the play of spectral colors that diamonds reflect.
D-F: Diamonds in the color grades of D, E, and F are considered colorless. Only by direct comparison to a D can a very slight difference in E and F be discerned. Diamonds in this color range are very rare.
G-J: Diamonds in the color grades of G, H, I, and J are considered near-colorless. Diamonds rated G, H, and I appear colorless in their settings and must be compared to diamonds of higher grades to discern any slight tint of color. A diamond with the color grade J may appear to have a very slight tint of color in its setting, but it should cost less than diamonds with higher color grades.
K-M: Diamonds with color grades of K through M may appear faintly yellow, brown, or gray in their settings. These diamonds are much less valuable than diamonds in color grades D through J.
N-Z: Diamonds with color grades of N through Z have an apparent, even distracting, yellow, brown, or gray tint in their settings. These diamonds are significantly less valuable than diamonds in higher color grades.