The impurities trapped inside the diamonds when they are formed are what give yellow diamonds their colour. The GIA diamond colour grading scale goes from D to Z. The diamonds towards the Z end of the scale have a more yellowish tone the further down the scale you go. Although paler yellows are more common, they are not as desirable as the darker yellows. The intense fancy yellow diamonds are the rarest of the yellow diamonds, and are the most valuable. Only about one in every 16,500 carats mined is graded as a fancy vivid yellow diamond.