A diamond's cut is not only about its shape, but how effectively the stone can return light back to the viewer's eye. A well-cut diamond will appear very brilliant and fiery, while more poorly cut stones can appear dark and lifeless, regardless of color or clarity.
Not only do well-cut diamonds appear more brilliant, they can also appear larger than other stones of the same carat weight. An ideal stone has both increased brilliance as well as increased diameter relative to more deeply cut diamonds.