Cobalt-Coated Sapphire
Recently, the New York laboratory received a bright blue stone for identification. The blue color closely resembled that of the highly desired cobalt spinel. Advanced testing procedures of Fourier-transform infrared (FTIR), ultraviolet/visible/near-infrared (UV-Vis-NIR), energy-dispersive X-ray fluorescence (EDXRF), and laser Raman spectroscopy were performed to aid in identification. FTIR revealed a corundum spectrum, but with an uncharacteristic rise in the 7000–5800 cm–1 region. Laser Raman spectroscopy confirmed corundum, and UV-Vis-NIR yielded a cobalt absorption spectrum similar to that of diffused cobalt spinel.
Magnification revealed the presence of a coating, which was responsible for the observed over-the-limits RI reading. The coating was easily detected by the speckled, uneven surface and lighter facet edges where the coating was damaged (figure 1). EDXRF identified the coating as cobalt, consistent with the UV-Vis-NIR absorption spectrum. The stone possessed typical heated sapphire inclusions, such as altered fingerprints and frosted crystals. In addition, Raman spectroscopy identified glass-filled surface-reaching cavities (figure 2).
Cobalt coating is already a known treatment of tanzanite to improve the blue color component, but GIA has not observed cobalt coating on sapphire in recent years (see Spring 1994 G&G, pp. 48–49). Unlike cobalt coating on tanzanite, which has been observed applied only to the pavilion, this coating covered the entirety of the sapphire. This highlights the possibility of cobalt coating on materials other than tanzanite to induce a more pronounced blue color.
Because of its color, this material is most likely to be mistaken for cobalt spinel, which can easily be eliminated as a possibility through standard gemological testing with the polariscope and specific gravity. The stone tested was doubly refractive and had a specific gravity of 4.00, while spinel is singly refractive and has an SG of 3.60 (+0.10/–0.03). Although the RI was over the limit of the refractive index liquid, the stone’s specific gravity and doubly refractive nature point to sapphire. Advanced testing was useful in confirming the true identity of the stone and the coating.
