GIA's SYMMETRY GRADING BOUNDARIES FOR ROUND BRILLIANT CUT DIAMONDS

摘要

Since 2006, GLA has used certain proportion mea- surements obtained with non-contact optical scanners to grade the cut of round brilliant diamonds. Improvements in the operation and accuracy of these instruments now enable us to also measure some symmetry parameters during the grading process. Although both Excellent and Very Good symmetry grades meet GIA's criteria for an Excellent cut grade (Moses et al., 2004), there is a premium for what the trade calls a "triple Excellent": an Excellent grade for cut, polish, and symmetry. Therefore, many diamond manufacturers would like to be able to predict GIA symmetry grades from measurement data, so they can apply these consider-ations during planning and cutting. Likewise, makers of non-contact optical scanners have been interested in guidelines for how measurable symmetry parameters affect the GIA symmetry grade. The grade boundaries presented here offer a substantive estimate of the symmetry grade for any round brilliant cut diamond.