Rating Criteria for the Proportions of Round Brilliant Cut Diamonds

In the evaluation content of round brilliant cut diamonds, the evaluation of degree of modification (including facet symmetry and polishing quality) is not controversial, and the focus of controversy is mainly on the proportion of round diamonds (including table width ratio, Height ratio, girdle-thickness ratio, pavilion-depth ratio, culet ratio, total depth ratio, star facet length ratio, lower girdle length ratio). On the issue of the proportion of round diamonds, there is no common standard that everyone agrees on. In different countries and regions, the standards implemented in diamond grading are different, so there are many "ideal cuts". 

 1. American Ideal Cut The American Brilliant Cut was designed in 1919 by American Marcel Tolkowsky, the founder of the modern round brilliant cut, through mathematical calculations based on optical principles. The standard proportions are: platform width ratio 53%, crown height ratio 16.2%, crown angle 34°30', pavilion depth ratio 43.1%, pavilion angle 40°45'. 

 2. Practical perfect cut Practical Fine Cut was designed and invented by German W.E. Eppler in 1949. The standard proportions are: platform width ratio 56%, crown height ratio 14.4%, crown angle 33°10’, pavilion depth ratio 43.2%, pavilion angle 40°50’. At present, in Europe, diamonds of better quality are mostly processed into this cut shape. Therefore, this cut is also known as the European Fine Cut. 

 3. International Diamond Council cut The IDC Cut is designed and launched by the International Diamond Council. The standard proportions are: platform width ratio 56%~66%, crown height ratio 11.0%~15.0%, crown angle 31°0'~37°0', pavilion depth ratio 41.0%~45.0%, pavilion angle 39°40' ~37°0'. 

 4. Scandinavian cut Scandinavian cut (ScanD.N.Cut), in 1970, was designed and launched by the Scandinavian Diamond Council. Its standard proportions are: platform width ratio 57.5%, crown height ratio 14.6%, crown angle 34°30’, matter depth ratio 43.1%, pavilion angle 40°50’. 

 Due to the inconsistency of the standards of the above-mentioned cut shapes, among the different diamond grading systems in the world, the differences in the cut evaluation are the most. In the grading standards of IDC and HRD cut ratio, the grade evaluation rules of very good, good, general (or medium), and poor are proposed.

 GIA proposes 1, 2, 3, and 4 grade evaluation rules according to the cut ratio and modification; 

 CIBJO does not have a similar grading regulation, and tends to describe the proportions and finishes of diamonds, but does not evaluate them. Although there is no consensus on the optimal proportions of round diamonds, there is consensus on poor ratios. For example, the "fish-eye stone" with a shallow pavilion and the "black bottom stone" with a deep pavilion are both typical examples of poor proportions. Because the brilliance of this type of cut diamond is greatly affected, it should be noted in the remarks when evaluating the diamond.