corundum, orange sapphire, synthetic corundum.7, gems

Magnetic resonance spectroscopy is investigated as an additional means of distinguishing natural from synthetic sapphires in the padparadscha colour range when microscopic examination is inconclusive. Natural stones from Sri Lanka and Tanzania are compared with Verneuil flame-fusion material and a Chatham flux-grown synthetic sapphire. Measurements were performed at room temperature using a microwave frequency near 9.2 GHz. Spectra were recorded with the applied static magnetic field perpendicular to the crystal’s trigonal axis for every sample and parallel to that axis for selected specimens. All examined Sri Lankan natural sapphires showed readily detectable ferric iron (Fe³⁺) signals together with varying amounts of trivalent chromium (Cr³⁺); chromium increased as the colour changed from apricot towards pink. Tanzanian specimens likewise displayed strong ferric iron signals, but their resonance lines were substantially broader than those of the Sri Lankan material, and chromium appeared relatively less abundant. The authors interpret the greater iron signal strength and linewidth in the Tanzanian stones as evidence of higher ferric iron concentrations, suggesting that these characteristics may assist geographical origin determination. In the flame-fusion synthetics, ferric iron signals were absent or very weak, with inferred amounts approximately one hundredth of those in the natural comparison stones. Consequently, a sapphire of padparadscha hue containing little or no detectable ferric iron is considered very likely to be flame-fusion synthetic. The Chatham flux-grown specimen contained both ferric iron and chromium and broadly resembled the natural spectra, but its resonance lines were markedly narrower than even those of the best Sri Lankan comparison stone. This sharpness permitted observation of normally “forbidden” transitions in the ferric iron spectrum and hyperfine structure associated with chromium, reflecting a high degree of crystal lattice perfection. Comparison of parallel spectra provided further confirmation of the distinction. The results support magnetic resonance spectroscopy as a method for separating the examined natural and synthetic materials, differentiating synthetic growth processes, and providing indications of natural provenance. The authors acknowledge the small number of stones studied and recommend using the technique within a battery of gemmological tests, particularly where more readily available methods leave uncertainty.