The diamond colour scale was devised by the Gemological Institute of America (GIA) in the mid 1950's to define and regulate the colour spectrum of diamonds. The GIA proposed starting at D as the best and most colourless diamond available. The colour D was chosen because diamond dealers were already using A, B and C and a break was required to regulate and harmonize the industry. The diamond colour scale progresses through the alphabet from D until Z. The graphic below illustrates the GIA colour scale.
D - E - F
Colourless
G - H - I - J
Near Colourless
K - L - M