Cut is responsible for the sparkle of the diamond and many believe that it has the greatest effect on the diamond's beauty. Superior cut reveals the fire and the brilliance of the diamond. A diamond with great light performance is extremely eye-catching and can even look bigger than its actual carat size.
Cut is the only one of the 4Cs directly affected by human skill and expertise. In order to maximize the fire and brilliance, the diamond cutter must place each of the stone's facets in exact geometric relation to one another. This geometric relation is the basis for the diamond cut grading.
The American Gem Society evaluates diamond cut on a scale of 0 (zero) to 10, where zero is the highest grade, and 10 is the lowest.
When Gemological Institute of America uses an 'Excellent', 'Very Good', 'Good', 'Fair', 'Poor' grading scale.
On a classic round brilliant-cut diamond, fifty-eight facets must be precisely aligned. When a diamond is cut too deep or too shallow, light leaks out of the bottom of the diamond, instead of reflecting back to you.This explains why some diamonds are so much more brilliant than others.