Diamond colour is all about what you can’t see. Diamonds are valued by how closely they approach colourlessness: the less colour, the higher their value. (The exception to this is fancy colour diamonds, such as pinks and blues, which lie outside this colour range.) Most diamonds found in jewellery shops run from colourless to near-colourless, with slight hints of yellow or brown.
GIA’s colour-grading scale for diamonds is the industry standard. The scale begins with the letter D, representing colourless, and continues with increasing presence of colour to the letter Z, or light yellow or brown. Each letter grade has a clearly defined range of colour appearance. Diamonds are colour-graded by comparing them to stones of known colour under controlled lighting and precise viewing conditions.
Many of these colour distinctions are so subtle as to be invisible to the untrained eye. But these slight differences make a very great difference in diamond quality and price.
Why does the GIA colour grading system start at D?
Before GIA developed the D-Z Colour Grading Scale, a variety of other systems were loosely applied. These included letters of the alphabet (A, B and C, with multiple A’s for the best stones), Arabic (0, 1, 2, 3) and Roman (I, II, III) numerals, and descriptions such as “gem blue” or “blue white”. The result of having all of these grading systems was inconsistency and inaccuracy. Because the creators of the GIA Colour Scale wanted to start fresh, without any association with earlier systems, they chose to start with the letter D, a letter grade normally not associated with top quality.