Not entirely true...David Atlas has had some charts up a long time that were pretty accurate. The charts are down but replaced with a calculator:
http://datlas.com/AGA_NAJA_Cut_Class_grader/index.php
That stone got an overall 4A rating which is below average.
1A-1B are American Ideal Cuts
2A-2B are International Fine Trade Cut
3A-3B are US Domestic Average Cut
4A-4B are Below Average Cut
There is A LOT of good information on that site.
That doesn't have a drop down for cushion cut. Again, some might rely on number to determine a degree of cut quality, however outside rounds and a much lesser extent princess cuts, it is next to impossible to judge a diamonds performance(light return, scintillation, etc) based solely on numbers.
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