Influence of Locality on Colour Enhancement of Heat- Treated Milky Geuda from Sri Lanka; A Mineralogical and Geochemical Investigation

Abstract

Blue sapphire is one of the most important gemstones in Sri Lanka and has a high commercial value in the gem industry. Geuda is a low-grade variety of corundum that can be converted into blue sapphire through heat treatment. Nevertheless, Geuda from different localities exhibit variations in chemical composition, inclusions, and structural features, which may influence colour enhancement during heating. Therefore, this study investigated the effect of the locality of Milky Geuda on colour enhancement after heat treatment. A total of 53 Milky Geuda samples were collected from several gem-bearing regions in Sri Lanka including Kataragama, Ridiyagama and Bakamuna. The samples were analysed before and after heat treatment using optical microscope, UV-Visible spectroscopy, Fourier Transform Infrared (FTIR) spectroscopy, Raman spectroscopy, and X-ray Fluorescence (XRF) spectrometry. Visual observations revealed significant colour enhancement in most samples, while only a few showed slight changes. Transparency has been improved in all samples after heat treatment. Fractures and cracks have been increased due to the thermal expansion of inclusions. The highest number of fractures was observed in samples from Kataragama, whereas Ridiyagama samples showed the lowest occurrence. UV-Visible spectroscopy indicated that Bakamuna samples retained Fe3+ absorption peaks at 388 nm and 450 nm after heating, suggesting weaker blue colour development. FTIR analysis showed the presence of OHfeatures before heating, which disappeared or changed after treatment. Raman spectroscopy demonstrated locality-dependent peak shifts, with the smallest shifts observed in Ridiyagama samples and the largest in Bakamuna samples. XRF analysis revealed that increased concentrations of iron and titanium, together with reduced aluminium content, enhanced blue colour development. Higher vanadium concentrations contributed to darker blue hues. The optimum elemental ratio for effective colour enhancement was identified as Ti:Fe:Al = 1:3:323.

• Colour Enhancement
• Corundum
• Heat Treatment
• Locality Effect
• Milky Geuda