Discovering Colour Alteration Conditions and Inclusion Changes of Sri Lankan Black/Brown (Penithora/Maangu) Tourmaline
Black or brown tourmalines, commonly referred to as "Penithoora" or "Maangu" in Sinhalese, are predominantly found in Sri Lanka and are primarily used in the production of decorative items due to their limited market demand and lower value as gemstones. This study aims to investigate the conditions under which the color of black/brown tourmaline can be altered through heat treatment, with the objective of maximizing profitability from gem pits. A total of sixty samples of black/brown tourmaline were subjected to heat treatment using a "Lakmini" gas-fired furnace, employing five different temperatures (700 °C, 750 °C, 800 °C, 850 °C, and 900 °C) and varying soaking times of two, three, and four hours under oxidizing conditions within the gas furnace. The samples were subsequently analyzed using Energy Dispersive X-Ray Fluorescence (ED-XRF) spectroscopy and a gemological microscope to assess changes in chemical and optical properties. The results obtained from ED-XRF spectrometry indicated that silicon, aluminium, and magnesium were the major elements, while titanium, iron, and manganese were significant trace elements present in black/brown tourmaline. Minor color reduction was observed at 700 °C, with slight brown color reduction occurring at 750 °C and 800 °C. Significant color reduction coupled with clarity enhancement was achieved at temperatures of 850 °C and 900 °C. By elucidating the color alteration conditions and associated changes in inclusions, this study contributes to the understanding of heat treatment effects on black/brown tourmaline, enabling enhanced identification and potential value optimization of these gemstones. Following the heat treatment, crystal inclusions exhibited a cloudy or partially melted appearance, resembling sugar crystals. Trichites, which exhibited a continuous flow in natural stones, appeared discontinuous after heat treatment. Therefore, heating at 850 °C for a maximum of three hours resulted in a more reduced brown/black translucent appearance with improved clarity.
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