Identifying the Cut Grade (5.44)
The most cost effective approach in recognizing proper proportions is with your 10x loupe. If necessary review these previous sections to understand the importance of the "Cut Grade": Note: Some subsections were intentionally left out since they do not directly relate to "Cut Grade".
The Pavilion Main Method (5.14)
Estimating Pavilion Angle (5.15)
Table Reflection Method (5.17)
General Rules for Pavilion Depth Estimation (5.18)
The Table Percentage Ratio Method (5.26)
The Table Percentage Bowing Method (5.27)
Estimating Table Size by Eye (5.29)
The Overall Appearance of the Girdle (5.35)
Upper and Lower Girdle Facets (Halves) (5.36)
The Role of the Upper and Lower Girdle Facets (Halves) (5.37)
Lower Girdle Facet Height (5.38)
Upper Girdle Facet Height (5.39)
Cutting the Upper and Lower Girdle Facets (Halves) (5.40)
Proportion Analyzers
The very popular Sarin DiaScan Diamond Cut Evaluator (currently selling for $7400-$9400), is a popular choice for diamond cutters and volume dealers. The Sarine Diamension HD is more sophisticated and detailed instrument. There are also other similar instruments from Sarine.
Left Courtesy of Kassoy: https://kassoy.com/sarin/sarin-diascan-diamond-cut-evaluator.asp
Right Courtesy of Aerodiam: https://aerodiam.com/sarin-products/diamension-hd/
The more affordable (on the preowned market), old school GIA Proportionscope:
GIA's Facetware: Free access with (login credentials) to GIA's database of 38.5 million proportion possibilities. https://www.gia.edu/facetware
Courtesy of Lexus Group https://lexusindia.in/so-gf.html
General Proportions and Characteristics
The general proportions and characteristics you will look for in an excellent cut grade for a round brilliant cut are:
Table Size: 53%-60%
Crown Angles: 33.7-35.8 Degrees
Girdle Thickness: Thin-Slightly Thick (No wavy or scalloped girdles. Make sure girdle is parallel with table).
Pavilion Angle: 40.2-41.25 Degrees (Consistent angles all the way around. Watch out for out of center culets). Use the Table Reflection Method.
Total Depth: 59.0%-62.0%
Culet: None (pointed) or Very Small (No abraded culets or very small culets that do not form a perfect octagon).
Polish and Symmetry: Excellent or Very Good