The colour palette of diamond is richly varied. It ranges from pink through red, green, blue, yellow, brown and black. The rarest colour occurring in natural diamond is red, followed by green, blue and purple. Colour occurs because of the existence of trace impurities. Brightly coloured diamonds, also known as fancies, are rare and valuable. The majority of diamonds occur in less popular hues ranging from pale yellow to brownish yellow (trace nitrogen). Pure white diamonds (ie colourless) are also rare, and highly prized. In fact diamond is the only gemstone whose colourlessness renders it more valuable. The GIA (Gemological Institute of America) pioneered the universal grading system in the early 1940’s to supersede all confusing grading systems in use at that time. The GIA colour grading classification starts from D, the finest colour (colourless), and proceeds through the alphabet to z getting progressively more yellow. A set of master comparison stones calibrated to the exact colour of every-other colour: E, G, I ... is the most reliable method of grading diamonds for colour. When compared in a standard viewing box or white folded paper the master stones are placed on either side of the diamond until the stones are sorted into the proper descending colour values. Diamonds are finally reassessed face-up for the most accurate, consistent colour grading.