White diamonds are not all colorless. They fall on a color spectrum, ranging from light yellow to totally colorless.
A lettering system from D to Z is used to identify the amount of color present in each diamond, with D awarded only totally colorless diamonds.
A colorless diamond can be graded D, E, or F; near colorless, G, H, I or J. The closer a white diamond is to being colorless, the rarer it is and the higher its value.