COLOUR: When we speak of Colour in a diamond, we actually mean the degree of colourlessness. While most diamonds are white, not all are truly colourless. Many are tinted yellow to brown or silver to grey. In a white diamond, the presence of a tint is considered undesirable. This colour impurity is caused by lingering traces of nitrogen, boron, hydrogen or other elements. Commonly, diamonds are affected solely by nitrogen traces, which create pale yellowish or brownish tints. Only diamonds composed of 100% pure carbon without any impurities may be completely colourless.
Diamonds are graded on a Whiteness scale or absence of colour scale. Basically, the whiter or clearer the colour of a diamond the greater its value.

GRADING: The Gemological Institute of America (GIA) has devised a set of guidelines to grade diamond colour. The colour of graded diamonds is compared to that of control stones, which are preselected gems of a specific colour.
To be graded, diamonds must be loose stones, because once a diamond is set into metal the metal can affect its colour. Diamonds are placed table-down, pavilion-up and viewed with a 10X loupe under controlled lighting and precise viewing conditions. A lettering system from D to Z is used to identify the amount of colour present in each diamond, with D awarded only to rare, totally colourless diamonds. Many of these colour distinctions are so subtle that they are invisible to the untrained eye. But these slight differences make a very big 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 all 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.

Size Description
D Absolutely colourless. The highest colour grade, which is extremely rare.
E Colourless. Very negligible traces of colour can be noticed by an expert gemmologist. A rare diamond.
F Colourless. Very negligible colour traces can be seen by an expert gemmologist, but still considered a "colourless" grade. A high-quality diamond.
G-H Near-colourless. Colour noticeable when compared to diamonds of better grades, but these grades offer excellent value.
I-J-K Colour slightly detectable. An excellent value.
L-M Noticeable colour.
N-Z Noticeable colour.