Modern colour grading system
The colour of a diamond is measured on an alphabetical scale beginning at D (Colourless) and ending at Z (Fancy Yellow Range).
But have you ever wondered why the alphabetical scale starts at D and not at A?
At the height of the Kimberly diamond rush a variety of methods were used to record the colour of a diamond. Some used multiple As for the clearest diamonds (e.g.: AAA, AA, A), others used Roman numerals (e.g.: I, II, III, IV), numbers (e.g.: 1, 2, 3) and gem-quality descriptions (e.g.: blue white, fine white), etc.
While these colour grading methods worked among local traders, discrepancies arose when the diamonds were traded internationally as different dealers used different grading systems. One dealer would classify his diamond as a AAA, while another would grade it differently depending on which method they adopted. This made it difficult to describe and value the diamond accurately.
As such, the Gemological Institute of America (GIA), an independent diamond grading laboratory, introduced a new universal scale and to avoid any further confusion, they started fresh at D. This is the modern colour grading scale and has been adopted world-wide.