A diamond's colour has a significant impact on its appearance. With its many prismatic facets, a colourless diamond reflects light into a spectrum of hues, a characteristic often referred to as "fire". The presence of noticeable colour in a diamond may reduce its ability to reflect light. Consequently, diamonds with lower colour grades will not show the same luminosity and fire as those with higher grades. The most valuable diamonds have little to no detectable colour.
The diamond colour scale begins at D (colourless) and ends at Z (light yellow). Diamond colour is often difficult to discern when a diamond is viewed face up. Therefore, gemologists typically evaluate diamonds facedown against a pure white surface, illuminated by carefully controlled lighting. The diamond is then compared to master stones of predetermined colour.
Highest-quality colour grade a diamond can receive. A D-colour diamond is extremely rare and emits unrivaled brilliance.
Contains very minute traces of colour. Also a rarity, an E-colour rated diamond emits a high level of brilliance and sparkle.
Minute traces of colour can only be detected by a trained gemologist. This is a high-quality colour grade.
Contains noticeable colour only when compared to higher colour grades. Appearing colourless to the untrained eye, a G or H colour diamond provides an excellent value.
Contains slightly detectable colour. An I-colour or J-colour diamond is an excellent value, as it typically appears colourless to the untrained eye.
Faint colour is noticeable. Even with the presence of colour, these grades of diamonds can emit fire and beauty.
Diamonds that exhibit a colour other than light yellow or brown, as well as diamonds that possess a more intense yellow or brown than the 'Z' colour rating, are considered "fancy coloured". With only one out of ten thousand diamonds possessing a natural colour, these types of stones are extremely rare. Depending on the colouration, intensity, and hue of a diamond, colour can either detract or enhance its value. Naturally occurring diamond colourus include gray, white, blue, yellow, orange, red, green, olive, pink, purple, brown, and black. Red stones are the most rare of the fancy coloured diamonds.
Similar to the process for grading colourless diamonds, a diamond's fancy colour is graded by comparing it to master stones of predetermined colour. Unlike colourless diamonds, fancy coloured diamonds are graded face up. The most important factors in determining the value of a naturally coloured diamond is the strength of its colour. The value of a fancy coloured diamond increases with the intensity of the most prominent hue within the stone.