In 1992 I discovered an inverse relationship between a diamond's crown and pavilion angles (the top and bottom facet angles). Our work with diamond cutters has given them new freedom to vary proportions to suit the rough diamond and achieve a beautiful gem. The shallow stone on the left has more light return and the steeper stone on the right shows more fire. I named them BIC - Brilliant Ideal Cut and FIC - Firey Ideal Cut.
The overlain profiles show how similar the light paths are, even though the angles vary considerably.
This is common sense; but common sense is not common. Many labs use a minimum - maximum crown and pavilion angle based grading system that penalizes cutters who produce such diamonds. Fortunately this is all now changing; the American Gem Society Lab is adopting the inverse proportion approach from the 1 st of June 2005 . The GIA is likely to follow soon.
If you know a diamonds proportions, you can use Holloway Cut Advisor (HCA) to check if it has ideal
The crown (top) angle on the orange FIC profile is 10 degrees steeper and the pavilion (bottom) is 2 degree shallower than the blue BIC stone, yet the light ray paths have similar paths. The variations compensate for each other.