The cut of a diamond is related to its proportion and finish. It affects the overall appearance and gives the diamond its sparkle. In January 1, 2006, GIA instituted a diamond cut grading system for Round Brilliant diamonds. This system is evaluates the loose diamond's overall appearance by using computer modeling and other methods to predict the levels of brightness, fire, and scintillation. Features such as weight ratio, durability, polish and symmetry are all included it the GIA cut grade. GIA cut is graded on a scale from Poor to Excellent. #1 Diamond Source sells GIA graded loose diamonds with "Good" or better cut grades.
A diamond gets its brilliance and scintillation by the cutting and polishing of its facets, allowing the maximum amount of light that enters through its top to be reflected and dispersed back through the top. With proper cutting (see diagram below) the light passes through the top, bounces off the sides, and then travels back out the top, giving the diamond optimum brilliance. If the diamond is cut too deep, light passes through the side of the diamond. If the diamond is cut too shallow, light passes through the bottom of the diamond, also inhibiting maximum brilliance.
Two popular overall proportion indicators are Total Depth Percentage (D%) and Table Percentage (T%) . D% is the diamond's depth expressed as a percentage of its width (average diameter for rounds). T% is the diamond's "table" width expressed as a percentage of its overall width (diameters for rounds). With non-round shapes ("fancy" shapes), much greater proportion variations are encountered. In most fancy shapes, higher D% and T% are more common and are dependent on their Length to Width ratios.
* Only in reference to Fancy Shapes and depends on personal taste.
** Most Significant with high colors (actually helps lower color diamonds).