In white or colourless diamonds, colour is universally graded using an alphabetical scale from D to Z with D being the highest grade.
Before the GIA colour grading system was universally accepted, various other systems were used including letters of the alphabet starting at A, B and C with multiple A’s used to grade the best stones, number systems and Roman numerals and the result of all of these grading systems was confusion and inaccuracy and so, to make a fresh start and to avoid any association with previous systems it was decided to us D as the highest grade.
The grader will assess the colour of the diamond using a master set of diamonds of each colour grade. Without comparing diamonds side by side it is very difficult to see the difference between a D colour and a G colour stone.
Stones graded D,E,F are described as colourless, G,H,I,J are near colourless, K,L,M are faint.