corundum, synthetic corundum.7, gems

Flux treatment of synthetic ruby is reviewed, and a previously uncharacterised inclusion pattern is described in stones encountered in the gem trade, often represented as natural rubies from Afghanistan. Conventional treatment involves immersing Verneuil flame-fusion synthetic ruby in an aluminium oxide-bearing flux, which penetrates fractures and promotes partial healing, leaving residual flux arranged in fingerprint-like patterns that can resemble healed fractures in natural ruby. Such treatment can also obscure curved growth striations. Products associated with Chatham, Knischka, and Lechleitner are discussed, although the authors note that preliminary heating and quench cracking have not been experimentally demonstrated for every product. Treatment may produce a flux-grown synthetic ruby overgrowth on the Verneuil core; remnants can reveal distinct boundaries, parallel growth lines, coarse flux inclusions, or small synthetic corundum crystals. Identification may depend on curved growth striations, gas bubbles following curved growth layers, or trace-element detection of molybdenum, lead, or tungsten derived from the flux. Since 1989, the authors had encountered approximately 60 treated synthetic rubies with a different inclusion structure. Twenty were examined by standard gemmological methods, and two, weighing 1.99 and 9.93 carats, underwent scanning electron microscopy with energy-dispersive spectroscopy and energy-dispersive X-ray fluorescence analysis. These stones contained thin, continuous, three-dimensional networks of flux-filled fractures, producing a honeycomb-like cellular appearance in transmitted light and whitish to colourless surface-reaching networks in reflected light. Their refractive indices, specific gravities, ultraviolet fluorescence, and absorption spectra fell within ranges that overlap those of natural and synthetic ruby. Extremely weak curved striations were resolved in one sample, but were insufficient to establish Verneuil growth conclusively; Czochralski growth remained possible. Analysis of the fracture fillings detected calcium and lead but no aluminium. The authors infer treatment in a flux possibly containing lead fluoride and calcium fluoride, while emphasising that the precise procedure remains unknown. The absence of dissolved aluminium oxide, insufficient treatment duration, or a different treatment process could account for the lack of conventional partial-healing patterns. Unlike the discontinuous fingerprints typical of earlier treated synthetics, the continuous cellular networks generally do not resemble natural ruby inclusions and permit ready identification with a gemmological microscope or even a hand lens.