Unusually Large Flux-Grown Synthetic Sapphire
IIGJ-Research & Laboratories Centre (formerly Gem Testing Laboratory) in Jaipur, India, recently received for identification a translucent blue rough stone weighing 64.70 ct. The specimen appeared transparent in transmitted light and displayed a typical bipyramidal (barrel-shaped) crystal habit along with distinct stepped striations across the surface (i.e., perpendicular to the longer c-axis, as shown in figure 1, left). In addition, some areas displayed a rhomb-shaped step pattern, and one side of the bipyramidal crystal had a large rhombohedral face (figure 1, right). This combination of crystal habit and surface features suggested corundum, which was confirmed by Raman spectroscopy.
Under magnification, the specimen displayed white and opaque flux residues in the form of wispy veils or flux fingerprints, along with flux-filled cavities (figure 2). No other inclusions associated with natural or synthetic corundum were observed. However, the specimen exhibited strong color zoning when immersed in methylene iodide; blue color was mainly concentrated toward the center of the stone, while the rim of the crystal appeared colorless (figure 3). The presence of flux (in the form of fingerprints/cavities) and the absence of any natural inclusions pointed to a synthetic sapphire grown by the flux-fusion process. Under short-wave UV, the specimen displayed strong chalky yellowish green fluorescence while remaining inert under long-wave; such a fluorescence reaction is not observed in natural (heated or unheated) sapphire. Closer examination in the DiamondView revealed stronger yellowish green fluorescence from flux inclusions compared to the stone’s body, although zonal greenish fluorescence was also visible. The exact cause of this yellowish green fluorescence remains unknown.
Consistent with an earlier report (Winter 2020 Lab Notes, pp. 524–525), this synthetic sapphire also displayed peaks at ~388 and 450 nm, along with a broad band at ~475–800 nm and an absorption maximum at ~615 nm in its ultraviolet/visible/near-infrared spectrum; these features are associated with Fe3+ and Fe2+-Ti4+. Semiquantitative energy-dispersive X-ray fluorescence analysis revealed the presence of molybdenum, tantalum, platinum, and silver, while vanadium and gallium were not detected; this chemistry further indicated a flux-grown synthetic.
This was the first time we had encountered such a large flux-grown synthetic sapphire, though the market availability of this size is unknown. Through careful microscopic observation, along with fluorescence reaction and chemistry, a laboratory gemologist should be able to correctly identify this type of flux-grown synthetic sapphire. But these sapphires, especially in their rough form, may pose a challenge for gem dealers who are generally equipped with a 10× loupe or only basic gemological tools.
