Colour diamonds, on the other hand, have proved much more resistant. That’s down to a number of factors, not least of which is the rarity of colour diamonds relative to colourless ones. Only around 0.01% of gem-quality mined diamonds are colour diamonds of the “fancy” type (a measure of the intensity of colour in the Gemological Institute of America [GIA] grading system). So, tracking valuations for colour diamonds is tricky. The same goes for red diamonds, which are really just an extension along the colour scale of pink diamonds.