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Definition: Colour, when applied to white diamonds, is often misunderstood by people outside the diamond industry.

Most people think that all white diamonds are colourless. In fact, truly colourless diamonds are quite rare. The diamonds used most often in jewelry are nearly colourless with faint yellow or brown tints. These diamonds fall into the normal colour range.Within the normal colour range, the closer to colourless, the more desirable and valuable the diamond.

In the 1950's, the GIA introduced the D-Z colour grading scale that is still the industry standard.

GIA Colour Grade Scale
D - F: Colourless
G - J: Near Colourless
K - M: Faint Yellow
N - R: Very Light Yellow
S - Z: Light Yellow

How is colour measured?
Colour in white diamonds is most commonly measured by comparing the diamond to a set of 'master stones' of known colour. The highest 'master stone' is E-colour. Any diamond better than the E-colour master is rated D-colour.

Each colour grade is actually a narrow range of colors. There is no one exact colour for any GIA grade. So, an F-colour diamond can be a strong, average or weak F-colour. However, as long as the diamond is better than the G-colour master, it receives an F-colour grade.

Diamond Colour Chart - Colours D to M

Fluorescence
Fluorescence is a unique effect that causes some diamonds to produce a glow (usually blue or yellow) when exposed to a strong ultraviolet light. Some prefer a diamond with some 'blue fluorescence' because it may make the diamond in the 'near colourless' or 'faint yellow' ranges look somewhat whiter.

In the 'colourless' range, fluorescence has no effect on colour, though it can cause the diamond to have a slightly milky appearance.

Why is a diamond's colour grade important?
The brilliant, fiery, sparkles of light that radiate from a high quality white diamond are unmatched by any other gem. The diamond acts like a prism, dividing light into spectral colors, which are then reflected as colour flashes, called fire. Any natural colour in the diamond filters the light, reducing its fire and brilliance.

The less colour in the diamond, the more colorful the fire, and the better the diamond's colour grade. The better the colour grade, the more desirable and valuable the diamond.