For grading purposes, the word "cut" means the proportions and finish-or make-of a polished diamond. Proportions are the angles and relative measurements of a polished gem's facets and the relationship between them. Finish is the quality of the polish and the precision or symmetry of the cut. These factors, along with the thickness of the girdle, affect a diamond's overall cut grade. Every angle and every facet affects the amount of light returned or lost.
The cut-grading system developed and used by GIA applies to standard round brilliant diamonds in all clarities across the D-to-Z color range. It assigns one of five cut grades: Excellent (Ex), Very Good (VG), Good (G), Fair (F), and Poor (P).
In the GIA system, the cut grade is determined by the diamond's proportions or finish. If one proportion factor is more extreme than the others, it can lower the cut grade of an otherwise well-cut diamond. An extremely large table facet, a very steep crown angle, a very deep pavilion, or thick girdle can reduce a diamond's cut grade significantly.