Feature Gems & Gemology, Spring 2012, Volume 48, No. 1

Precision Measurement of Inter-Facet Angles on Faceted Gems Using a Goniometer

Andy H. Shen, William A. Bassett, Elise A. Skalwold, Nicole J. Fan, and Yong Tao

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A classic two-circle reflecting goniometer was used to measure inter-facet angles on five faceted diamonds that included round brilliants and fancy shapes. The instrument provided significantly better precision (to within 2 minutes, or 0.034°) than the non-contact optical scanner that is customarily used at GIA for this purpose. With some procedural modifications, the goniometer could make measurements of all inter-facet angles, including the pavilion facets. The technique is potentially valuable for producing a well characterized set of reference stones for calibrating non-contact optical scanners.