For the following discussion please refer to the table on the right to visualize the diamond colour corresponding to each letter of the scale.
In general, the less colour a diamond has, the greater its value.
Colours range from colourless (D) to light yellow (Z).
Absolutely colourless, extremely rare diamond.
Still considered colourless, slight colour detectable by an expert. Also a rare diamond.
Colour is noticeable when compared alongside a colourless diamond.
Colour is slightly noticeable.
Increasing amount of colour, diamonds less valued than the ones above. After Z, as we move into the fancies, the diamond value starts to increase again.
Diamonds in this colour range are called fancies. The most rare and expensive fancy colours are Red and Green. Other colours include Purple, Orange, Pink, Yellow and Black. Fancy diamonds are graded on a different system with colour grades such as: faint, very light, light, fancy light, fancy, fancy intense, and fancy dark.
When mounted in jewellery, even G, H, I and J diamonds can appear colourless (especially when set in yellow gold as opposed to white gold). It takes a highly trained eye to notice the slightly yellow/brown tinge in them. For diamonds less than half a carat that have been mounted in jewellery it is almost impossible for the untrained eye to detect colour up to I. However the colour is there and has an impact on the value of the diamond.
Colourless diamonds are extremely rare, and hence expensive, so most diamonds in jewellery are 'near colourless' with extremely faint yellow/brown tints. Despite the tint, 'Near Colourless' diamonds are considered to fall within the 'normal' colour range.
Diamond colour can be affected by treatments such as HPHT (High Pressure, High Temperature Treatment) and Irradiation. Fluorescence can also affect colour.
Fluorescence is produced when UV light (eg. from the sun) hits a diamond. It occurs in over 30% of gem-quality diamonds. The UV light causes excitation of electrons in the diamond crystal producing a usually blue light in varying intensities. When the UV source is removed so to does the effect stop. The reason you should know about fluorescence is that if strong it can alter the colour you perceive a diamond to be; ie strong fluorescence can make top range colours (D - H) appear slightly cloudy (a negative) whilst making lower grades (I - Z) appear to have less yellow colour than they actually do (a positive). When buying a Diamond that has fluorescence, make sure that it is blue and no other colour. If blue, it won't be a factor in the stone's appearance unless the intensity grade is strong or greater. Even in these cases it is rarely a problem and can in some cases be a positive as described above.
High Pressure/High Temperature (HP/HT) treatment With this annealing process diamonds of low colour can be converted to (D-H) colour. The technique has also been applied to Brown diamonds to yield other fancy colours. Research shows that diamonds that have undergone these treatments can be detected by sophisticated instruments.
This is a technique which uses a nuclear reactor to produce fancy coloured diamonds.