The cut grade rates how well a diamond's proportions and finish work, from Excellent to Poor. GIA gives it only to standard round brilliants in the D-to-Z color range.
In more detail
GIA's system went onto reports in January 2006. Seven components set the grade, and the lowest one decides. They are face-up appearance (how bright it looks, fire, scintillation), weight ratio, durability, polish and symmetry. For Excellent, each proportion has its own limits, and GIA also grades them in combination.
On a grading report
In the grades block of a GIA report for a D-to-Z round brilliant; absent for fancy shapes and fancy colors. IGI grades round cut Excellent to Poor with one published range per proportion. It also offers an optional cut grade for fancy shapes.
Related terms
- Weight ratio: Weight ratio is the part of GIA's cut grade that compares a diamond's weight with its diameter. A stone can carry too much or too little weight for its width.
- Polish: Polish is the grade for the smoothness of the facet surfaces, from Excellent to Poor. The grader looks at 10x for pits, nicks, polish lines, scratches, burn marks and rough patches.
- Symmetry: Symmetry is the report grade, from Excellent to Poor, for the exactness of a stone's outline. It also covers the shape, placement and alignment of the facets.
- Ideal cut: "Ideal cut" has three meanings: an IGI comment, a GIA supplement grade, or a sales word. At IGI it marks an Excellent round with narrower proportions; at GIA it appears only as "AGS Ideal".
- Proportion diagram: The proportion diagram is a side-view profile of the stone, drawn to its measured proportions. It shows the table, depth, crown, pavilion, star and lower half lengths, girdle and culet.
- Brilliance: Brilliance is the white light that a cut stone returns to the viewer. In our census, it is the share of entering light that leaves through the crown toward the viewer.