corundum, synthetic corundum.7, flux, Kashan, gems

An approximately 3 ct faceted ruby examined in 1975 is described as a previously unrecognized type of synthetic material whose conventional gemmological properties provided little conclusive evidence of origin. The stone had an eight-sided modified brilliant cut, saturated violetish red colour and violetish red and orange dichroism. Refractive indices of 1.764–1.772, birefringence of 0.008, specific gravity of approximately 4.02 and a typical chromium absorption spectrum were consistent with ruby. Ultraviolet excitation produced medium red fluorescence, while x-rays induced strong red fluorescence without visually detectable phosphorescence. Short-wave ultraviolet transparency was intermediate between that of the natural and flux-grown synthetic comparison materials. Attempts to determine the ultraviolet absorption edge spectrometrically were inconclusive because the stone’s strong colour and unsuitable geometry caused excessive absorption. Detailed microscopy, using higher magnifications, immersion and carefully arranged illumination, revealed the features on which identification principally depended. Minute bright particles, interpreted as probable cavities, occurred irregularly along approximately parallel planes. Some intersected polished facets, producing a finely pitted surface visible in reflected light. Coarser groups of roughly oriented cavities had rounded ends, occasionally forming dumbbell shapes, while isolated paired cavities and a few narrow straight needles, including one with a forked termination, were also observed. Immersion examination disclosed a wavy growth plane with irregular colour saturation and fine, straight traces of parallel growth planes throughout the specimen. Particularly significant was a localized comet-like distribution of minute particles, spreading from a rounded head into diverging flow patterns and forming hairpin-like structures where the lines converged. The author considered this combination inconsistent with known natural rubies and previously familiar synthetic products, with the flow pattern providing especially strong evidence of synthetic origin. Consultation with other laboratories established that comparable material had been encountered elsewhere and suggested manufacture in the United States, although the production method was not identified. The specimen’s earlier sale as natural ruby and an accompanying natural-origin report illustrate the identification difficulty. Photomicrographs document the diagnostic observations, but the paper stresses that superficial resemblance to illustrated inclusions is insufficient: difficult specimens require experienced interpretation and, where necessary, specialist laboratory examination.