Low Temperature Heat Treatment of Burmese Ruby

Detection of low-temperature heat treatment (below 1200°C) in ruby and sapphire can present challenges to gemologists, as alterations to the material are often subtle. In this study, the authors heated Burmese ruby samples from Mogok, Myanmar, at temperatures ranging from 600° to 1500°C. The samples were documented using macrophotography, photomicrography of inclusions, ultraviolet fluorescence imaging, and spectroscopic analysis (Raman, ultraviolet/visible/near-infrared, and infrared) to record any changes, with a focus on features that could help detect heat treatment. A wide variety of solid inclusions, including calcite (Figure 1), mica, spinel, and zircon, were found in Mogok ruby. Many of those were found to be sensitive to heat treatment with regard to morphology and phase transition; their reactions varied depending on a number of factors such as size, distance from the surface, and species. Microscopic examination provided useful visual indications of heat treatment, even at lower temperatures (Figure 2). Raman analysis of calcite and spinel inclusions (Figure 3) also proved valuable in providing complementary evidence of low-temperature heat treatment. Spinel inclusions in particular showed a widening of peak in the range of 405–420 cm–1, which is known to occur when spinels are heated (Saeseaw, 2009). Fluorescence images were also recorded before and after treatment (Figure 4). The combination of these factors can assist gemologists in detecting low temperature heat treatment.