corundum, synthetic corundum.7, gems

A Russian synthetic ruby submitted in 1993 to the German Foundation for Gemstone Research is characterised by standard gemmological testing, microscopy, visible absorption spectroscopy, and chemical analysis. Although reportedly produced hydrothermally in Novosibirsk, the specimen is identified as a product of flux synthesis. The investigation is placed within the expanding supply of Russian synthetic gemstones following the political and economic changes in the former Soviet Union, with a table summarising materials and production methods previously reported from the region. Earlier Russian rubies had included both flux-grown and hydrothermal products. The examined crystal weighed 138 carats, had a boule-like shape, and was dark red, showing distinct purplish-red and orange-red pleochroism. Macroscopic examination revealed dark dendritic inclusions and a metallic coating over a small surface area. Measured refractive indices were 1.762 and 1.770, with birefringence of 0.008 and specific gravity of 4.00. Fluorescence was strong red under long-wave ultraviolet radiation and weak red under short-wave radiation. Immersion microscopy revealed opaque black flux residues with characteristic dendritic forms; dark-field illumination showed their polycrystalline nature. A second inclusion type consisted of clouds of bubble-like flux residues within partly healed cavities, resembling features described in Knischka synthetic rubies from Austria. Polarised visible absorption spectra displayed pronounced chromium-related lines and bands, including a line at 692 nm. Broad absorption maxima occurred at 545 and 409 nm for light polarised parallel to the crystallographic c-axis, and at 554 and 400 nm for perpendicular polarisation. Scanning electron microscopy with energy-dispersive X-ray analysis gave a composition of 98.61 wt.% aluminium oxide and 1.39 wt.% chromium oxide. Tungsten and molybdenum were detected in the metallic surface coating and in fractures where dendritic inclusions reached the surface. These flux-related elements and the diagnostic inclusion structures establish flux growth despite the reported hydrothermal origin. The physical properties and chemical composition otherwise fall within the ranges of synthetic rubies from other producers, making the microscopic and inclusion-chemical evidence central to recognising the material’s growth method.