The Gemological Institute of America grades the color from D alphabetically to Z. D being totally colorless, and Z yellow. The GIA considers the D, E, and F to be colorless, G to J is "near colorless", K - M faint yellow, M - R very light yellow, and S - Z light yellow. Diamonds of colour grade D are very rare, and extremely valuable.
Colourless and near-colourless diamonds are the most valuable. Though most diamonds may appear colourless to the naked eye, the majority of diamonds contain slight traces of yellow or light brown when viewed under a jeweller's loupe. Depending on the stone's size, a single increase in colour grade can boost the value of a diamond by thousands of dollars per carat. A traditional engagement diamond is usually colourless or near-colourless.
In nature, diamonds can also occur in shades of red, pink, blue, green and deep yellow - These are called "Fancy diamonds".
Always buy a diamond with a recognized Laboratory Report. Laboratory Reports from the major Gemological Institutes (IGI - International Gemological Institute, GIA - Gemological Institute of America) include a description and photograph of the jewellery item, along with other information such as cut, clarity, colour, and carat weight for diamonds, and dimensions in millimetres for any gemstones.