gems, corundum, synthetic corundum.7

Two bluish green faceted samples that resembled Paraíba-type (Cu-bearing) tourmaline (13.94 and 15.95 ct; Figure 36) were recently submitted to the DANAT laboratory for identification. Standard gemmological testing revealed RIs of 1.761–1.770 and a hydrostatic SG of 3.98 for both specimens, which match the values for corundum. The specimens were free of inclusions when viewed with 10×–70× magnification, suggesting a synthetic origin. However, no curved colour banding was observed with diffused lighting while they were immersed in water. As previously documented by Hughes (2019), curved banding in synthetic sapphire and ruby also can be revealed by exposure to short-wave UV radiation. We examined the present samples using a DiamondView, but they did not fluoresce under its ultra-short-wave UV source, nor under long-wave UV radiation. LA-ICP-MS chemical analysis of both samples was performed with an iCAP Q ICP-MS unit coupled with a Q-switched Nd:YAG laser-ablation device operating at a wavelength of 213 nm. The common trace elements in natural corundum—Cr, Ti, V, Fe, Ni and Ga—were all below the detection limits, which proved that the specimens were synthetic. (Detection limits were 1.8 ppmw Cr, 0.44 ppmw Ti, 0.75 ppmw V, 7.9 ppmw Fe, 2.1 ppmw Ni and 1.2 ppmw Ga.) Moreover, the analyses showed an elevated content of Co (51.1–78.1 ppmw).