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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
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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.