One of the 4Cs used to grade diamonds, colour in fact refers to lack of colour. The most highly prized and valuable stones are entirely colourless, clear as pure water. Diamonds are graded along a GIA scale, from D down to Z, the lowest grade. Alongside D, which is perfectly colourless, E and F grade diamonds are also classified as colourless, G-J are near colourless, while grades K-Z contain varying degrees of noticeable colour.