Blue Nile gemologists recommend selecting the highest cut grade within your budget. The reason is simple: of the Four Cs, no other characteristic has a greater influence on a diamond's appearance.
A diamond's cut grade is an objective measure of a diamond's light performance, or, what we generally think of as sparkle. When a diamond is cut with the proper proportions, light is returned out of the top of the diamond (which gemologists refer to as the table). If it is cut too shallow, light leaks out of the bottom; too deep and it escapes out of the side.