The Answer:

Diamonds with a color between colorless and yellow receive a letter grade on a scale that runs through the alphabet from D — colorless to Z — light yellow color. D is at the top of the scale, and is generally the most valuable. As you move down the color scale, the diamonds become more yellow in nature, and generally less valuable. (However, those at the very bottom of the scale have an appealing and rare "fancy yellow" color, which can be as valuable as colorless stones.)

There is a similar scale for brown diamonds, but these are generally not in demand by the public.

Here is a link to a diamond education page from Scheiner Diamonds, which describes the scales and their divisions in more detail, in addition to other criteria determining a diamond's value.

—The Editors