Colorless diamonds come in a variety of color grades that are marked D through Z. D, E and F grade diamonds are the most rare, and thus, the most expensive. These contain no color. The further down the scale you go, the more yellow appears in the diamond. However, it is very difficult for the untrained eye to see the differences in the stone unless they are viewed side to side. Remember, that a good diamond is a combination of the cut, color and clarity.