Lab Notes Gems & Gemology, Fall 2015, Vol. 51, No. 3

Coated Kornerupine Beads

Kornerupine beads with metallic coating.
Figure 1. These three kornerupine beads (0.95–1.38 ct) were found to have a metallic coating. Photo by Robison McMurtry.

Three dark brown faceted beads with an unusual metallic luster were recently submitted to the Carlsbad laboratory for identification service (figure 1). The three beads ranged from 0.95 ct to 1.38 ct. Standard gemological properties revealed an RI of approximately 1.668–1.680, consistent with kornerupine. The hydrostatic SG ranged from 3.01 to 3.19. This is slightly lower than expected for kornerupine, but can be explained by air trapped in the drill holes. Raman spectroscopy confirmed that the stones were kornerupine.

All three beads exhibited a high metallic luster with silver and bronze coloration, which is not typical for kornerupine. Under microscopic examination with reflected lighting, the surfaces showed spotty luster, consistent with a surface coating (figure 2). Further evidence of coating was visible along the facet junctions, where the coating was worn away.

Spotty luster on kornerupine bead.
Figure 2. Diffused reflected light exposed spotty luster on the surface of one of the tested stones, indicating the presence of a coating. Photomicrograph by Amy Neurauter; field of view 2.17 mm.

Energy-dispersive X-ray fluorescence (EDXRF) chemical analysis detected large amounts of Mg, Al, and Si, consistent with kornerupine. EDXRF also revealed the presence of copper. The authors could not find documentation of naturally occurring copper in kornerupine, so this element may have been used to create the metallic coating. This is the first time the GIA laboratory has seen coated kornerupine.

Amy Neurauter and Heidi Breitzmann are gemologists at GIA in Carlsbad, California.