corundum, synthetic corundum.7, gems

An unusual colour-change synthetic sapphire demonstrates that twin lamellae, although commonly associated with natural corundum, cannot independently establish natural origin. The specimen, examined at the GIA Gem Trade Laboratory’s West Coast facility in November 1994, was a transparent emerald-cut stone weighing 1.43 ct and measuring approximately 6.85 × 5.73 × 3.80 mm. It appeared bluish green under daylight-equivalent fluorescent illumination and pinkish purple under incandescent light. This behaviour resembled that of the vanadium-doped, flame-fusion synthetic sapphires frequently submitted to the laboratory, sometimes in the mistaken belief that they are natural or synthetic alexandrite. Such synthetic corundum is normally identified without difficulty by a vanadium-related absorption band near 474 nm and curved growth striae visible under magnification with darkfield illumination. In the reported specimen, initial binocular microscopic examination revealed neither solid mineral inclusions nor cavities such as gas bubbles or negative crystals. Instead, a conspicuous series of laminated twin planes was readily visible through the table facet. Although the author emphasises that twinning does not prove natural origin, its presence could have encouraged that interpretation because it is an uncommon feature in flame-fusion synthetic corundum. Continued examination from different directions resolved the apparent contradiction: observation through the pavilion facets revealed curved striae, establishing the stone’s synthetic origin. The note places this finding alongside earlier published accounts of polysynthetic twinning in Verneuil corundum and reviews other atypical internal features that can complicate identification. These include undissolved alumina, triangular cavities containing gas bubbles and needle-like inclusions. Their occurrence shows that synthetic corundum may contain features that differ from the familiar curved striae and rounded bubbles expected in routine examination. The principal diagnostic lesson is therefore methodological: an identification should not rest on the first inclusion or structural feature observed. All internal characteristics must be evaluated together, and the gemstone should be examined from several viewing angles before a conclusion is reached. In this case, the combination of unusual twinning with diagnostic curved growth striae illustrates the importance of a thorough microscopic investigation.