In the 1950s the Gemological Institute of America created the GIA Colour Grading Scale in order to standardise the practise and differentiate itself from existing nonstandard alphabetical grades being used at the time such as A, AA, AAA; A, B, C . It settled on the letter 'D' to define the most colourless diamond available, through to 'Z' at the scale's opposite extremity.

In order to obtain the highest rating, a diamond will be chemically pure and structurally perfect to give it it's transparency. In reality this is very rare, and only a tiny percentage will be colourless (D, E or F), or near colourless (G, H, I or J). Most diamonds mined are in the range of pale yellow or brown colour, which is known as the 'normal' colour range.

Yellow or brown coloured diamonds with more intense colour than 'Z' are considered fancy coloured diamonds alongside diamonds displaying other intense colours, such as pink, green or blue.

How diamonds are graded:

Diamonds in the normal colour range are graded loose (unmounted) with the table facet facing downward and the pavilion side facing upwards.

When a diamond is graded, its colour is compared to a masterstone set of diamonds. These masterstones exhibit the very least amount of body colour that a diamond in that colour grade may display. Against the masterstone, the grader classifies the stone as having more, equal or less colour and this determines the colour grade awarded.