 | The most valuable fancy colours are those with a single colour. They are difficult to find because they often have secondary colours. Grading a fancy colour is rigorous because they don't have the same number of points on a colour scale. When a fancy-coloured diamond is under evaluation, the GIA focuses on three areas:Hue - The colour (e.g. purplish-pink), Tone - How light or dark the colour is, and Strength/Saturation. |  |
 | Less than one in 10,000 gem-quality diamonds is classified as ‘fancy-coloured’. The various colours are formed under unique conditions involving specific trace elements, radiation and/or plastic deformation of the crystal structure. |  |
 | Fancy yellow and browns are more abundant. Rarest is the red diamond. Classified according to their hue, tone and saturation, the value of coloured stones generally increases with the strength and purity of the colour. Even very slight colour differences can have a profound effect on price. |
