GIA Brilliance Study Lacks Fire

After eight years of study, the Gemological Institute of America has issued the first of a series of reports on diamond cut (see accompanying article). The study offers some fascinating mathematical data. But with only one-third of the project complete, the report makes some hasty judgments on cut-grading systems already in place. GIA’s initial report literally lacks fire (discussion of spectral colors, or dispersion) and sparkle (discussion of brilliance and fire in motion, or scintillation). Using mathematical models and computer ray tracing, GIA’s researchers have unleashed an incredible mass of statistics on cutting for brilliance, measured in terms of “weighted light return” (WLR). The results prove that there are many ways to proportion a diamond for maximum brightness, all of them different from Marcel Tolkowsky’s original “Ideal cut.” As a diamond grader, I would

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