The Gemological Institute of America has established a colour scale that ranges from D (colourless) to Z (light yellow). Most diamonds are graded on how closely they approach colourless.

Colour differences are subtle, so it is very difficult to see the difference between, for example, an E and an F. For this reason, controlled lighting conditions are used to grade diamonds. They are also compared to a master set for accuracy.

Truly colourless diamonds, graded D, are treasured for their rarity. The choice of colour, however, is really a matter of personal preference.

Even more rare than colourless diamonds are those of Fancy Colour, for example blue, yellow or pink. The grade and value increase with colour intensity.