Diamonds are graded by laboratories for their lack of colour on a scale from 'D' colourless to tinted. Diamonds on this scale are valued for their lack of colour, with naturally formed colourless diamonds often being the most prized and highly valued gemstones. Diamonds are assessed unmounted, face down and against a white background under a daylight lamp by experienced diamond graders. Diamonds are assessed loose, as the metal of jewellery settings can affect the perceived body colour of the stone.
The colour in diamonds is caused by trace elements within the crystal structure. There are diamonds that have no other impurities within the crystal structure and these are colourless. The perceptible yellow colour in diamond lower down the colour scale is caused by nitrogen atoms within the diamond’s crystal structure.
The colour H is the part of the scale where most people notice a slight colour in the stone. However, this is incredibly difficult to detect.