The colour of a diamond depends on three variables: the light source, the object, and the response of the observer.
To standardize colour grading, diamonds are graded underbalanced (average) daylight-equivalent fluorescent white light between 5500 and mostly 6500 Kelvin colour temperature. This light range runs from what is the equivalent of a blue sunny sky to a uniform more overcast bluish sky. The lighting environment is very important and people should be aware that fluorescent lamps always intensify blue hues whereas halogen lamps often add yellow and incandescent lights red. The most neutral sunlight for general observation is usually experienced during mid-morning (very slightly yellow) or mid-afternoon as opposed to midday where blue is more prevalent or the later afternoon when yellow to red take over.
Diamonds are graded in a pure white colour tray, facing down by looking through their pavilion. White diamonds are judged according to their lack if inherent bodycolour, in other words for their ‘internal glow’. The background setting for grading should be neutral. The diamond to be graded is compared to master stones which sit at the bottom end of their respective colour grades. Diamonds are typically graded by up to three graders and more if result disputes occur. The actual colour letters describe a more or less narrow (colour) range of saturation.
Generally, the less colour a normal colourless ‘D-Z’ range diamond has, the rarer it is and the more valuable it becomes. A connoisseur of ‘white’ diamonds looks primarily for the return of white light or brilliance and the more colourless a diamond is the more brilliance it may have as colour always leads to greater light absorption and hence diminished light return.
The GIA has established a now internationally fully accepted colour grading scale. Old colour terms (eg. ‘Top Wesselton’) whilst sometimes still referred to, are no longer in use.
According to the GIA colour scale D-E-F (also called F+) stones are deemed colourless both face-down and face-up. They are often referred to as ‘collection’ colour or ‘fine white’ in the trade. Commercial white G-H-I-J are also known as near colourless, being colourless face-up and near colourless to slightly tinted face-down. K-L-M are termed faint yellow (or more often as light capes) and are slightly tinted face-up and obviously tinted face-down. Colours N through Z are very light or light yellow and show an obvious tint both face-up and face-down. All colours beyond Z are strictly considered to be fancy coloured diamonds.
This colour grading scale applies to yellow ‘cape’-series diamonds which derive their tinge from nitrogen impurities as well as grey and brown also called equivalent-grade (ECG) diamonds. In brown and grey stones however graders rather look at the tone than intensity as these colours tend to be more difficult to grade accurately. Another term sometimes seen is BGM, an abbreviation for ‘brown-green-milky’. Brown (see also ECG) and green refer to stones that have a touch brown or green, which again fall outside the case-series. These need to be assorted separately. Whilst not usually of lesser colour quality and despite their attribute of facing up whiter they may sometimes be cheaper due to an unwarranted trade ‘stigma’ attached to them. Milky stones, which are diamonds of afflicted clarity (as opposed to a colour), on the other hand, should always be avoided!