Round Brilliant Cut
The subject of debate for the past hundred years, Gemologists, Master Jewelers, diamond cutters, manufacturers, and resellers have all argued what the perfect proportions of a diamond should be; thus the various brands. The quality of the diamond’s cut is the most important aspect of the Round Brilliant cut, more so than color, clarity, and even carat weight. If your fiancée prefers a diamond that weighs more, an Oval cut, Cushion cut, or Pear cut may be more beneficial as they retain more rough weight from the original diamond crystal. A Marquis cut retains the most rough diamond weight of any shape.
A well proportioned smaller cut such as a 1.90ct as opposed to a 2.00 ct with Very Good Symmetry and Good Polish is recommended.
For more information on AGS Labs take on the Round Brilliant cut see: http://www.agslab.com/marketingpdfs/CuttingGuideforRounds.pdf
Generally Accepted Round Brilliant Diamond Parameters
Total Depth                57.5-63.8%
Table                           51.4%-62%
Girdle Thickness         Very Thin - Slightly Thick
Cutlet                          None-Medium
Polish                          Very Good
Symmetry                   VG-Excellent
Length to Width Ratio 1 to 1
Crown Angle              31.6-36.4 Degrees
Crown Height             11.1-18.1%
Pavilion Angle                        39.7-41.7 Degrees
Pavilion Depth           41.5-44.9%

Variations: The following are all supposed improvements to Tolkowsky’s original design from 1919: EX Royal Co. Royal Brilliant, Eightstar Hearts and Arrows, Gabi Tolkowsky & Sons Savannah, Genesis, Gerrard Eternal, De Beers Gabrielle, Leo Schachter Diamonds, Solasfera, Star129, Eighternity, Elara, and the Hearts on Fire.