gems, corundum, ultraviolet spectroscopy

The Krüss UVS 2000 ultraviolet spectroscope is evaluated as a relatively inexpensive aid for distinguishing natural ruby, blue sapphire and red spinel from their synthetic counterparts. Tests comprised 121 natural and 118 synthetic rubies, 109 natural and 26 synthetic blue sapphires, and 36 natural Burmese and 37 synthetic red spinels. The instrument directs light from a mercury lamp through the gemstone and a quartz prism onto a fluorescent scale, allowing the relative transmission of selected visible and ultraviolet emission lines to be compared. Correct positioning is essential because external facet reflections can produce misleadingly strong spectra. Two broad transmission patterns are distinguished: Type I, dominated by visible lines, generally characterises natural stones, whereas Type II, with relatively strong ultraviolet transmission around 315 nm, generally characterises synthetic material. Among the rubies examined, 116 natural specimens showed Type I spectra and 104 synthetic specimens showed Type II spectra. Exceptions included some flux-grown and weakly coloured flame-fusion synthetic rubies, together with occasional natural Burmese and Sri Lankan stones. Blue sapphire separation additionally depends on transmission at approximately 295 nm: most natural stones lacked this line, whereas synthetic sapphires commonly showed either Type II spectra or Type I spectra with the line present. Exceptions included three heated Sri Lankan sapphires and the single flux-grown synthetic sapphire tested. All natural red spinels showed Type I spectra, while 32 of 36 flame-fusion specimens showed Type II spectra; the single flux-grown synthetic spinel resembled natural material. Results were also weighted against the proportions of stones submitted to the AIGS laboratory, although the author acknowledges that these submissions were biased towards suspicious, relatively clean gems and did not constitute a random market sample. Differences associated with heat treatment were statistically significant for particular transmitted lines in Thai ruby and Sri Lankan sapphire, but were insufficient for conclusive treatment identification. Greater ultraviolet absorption in natural specimens is tentatively attributed primarily to chemical impurities, particularly iron. The author recommends ultraviolet spectroscopy as a supplementary test used with microscopy and fluorescence, with due regard for exceptions and sampling limitations. Ultraviolet spectrophotometry remains preferable for detailed analysis because it records a continuous spectrum, while the UVS 2000 offers lower cost. Heat and ultraviolet exposure from the instrument may damage sensitive stones or alter their colour.