Fair cut: A diamond that reflects light well and costs less than a Good cut. Represents roughly the top 35% of diamond quality based on cut. Still a quality diamond, but a fair cut will not be as brilliant as a good cut.
Cut Grading Systems analyze the light performance of a faceted diamond and measure the light rays traveling through all the facets in 3 dimensions instead of 2 dimensions.
And the cut grade rating determines the quantity and quality of light being returned to the viewer. In addition to proportion, symmetry and polish, brightness, dispersion, leakage, contrast and spread are also considered when assigning the final cut grade. Laboratories use the following attributes of a diamond to help determine the cut grade: depth %, table %, crown angle, pavilion angle, star length, girdle, polish, symmetry, and culet size. The Polygon Cut Grade feature applies to the standard round brilliant diamond cuts across all clarity ranges from the D-to-Z color-grading scale.