"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".
In more detail
IGI's May 2023 Ideal band starts with total depth 58.5 to 62.5% and table 53 to 60%. The crown angle must be 33 to 36° and the crown height 13 to 16.5%. The pavilion must be 40.0 to 41.3° and 42 to 44% deep, with a Thin to Slightly Thick girdle. Polish and symmetry must be at least Very Good.
On a grading report
IGI writes "Ideal Cut Round Brilliant" as a comment beside an Excellent cut grade. GIA prints no Ideal cut grade.
Related terms
- Cut grade: 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.
- AGS Ideal Report by GIA: The AGS Ideal Report by GIA is a digital supplement to a GIA report. It gives the American Gem Society's light performance grade, and only AGS Ideal 0 stones get one.
- Triple Excellent: Triple Excellent is trade shorthand for a GIA round brilliant with Excellent cut, Excellent polish and Excellent symmetry.
- Tolkowsky ideal cut: The Tolkowsky ideal cut is the round brilliant that Marcel Tolkowsky calculated in his 1919 book Diamond Design. It has a 40.75° (40° 45′) pavilion, a 34.5° (34° 30′) crown and a 53% table.