Identification of the Effect of Localization on the Colour Alteration of “Geuda” Gemstones Through the Heat Treatment Using ED-XRF, FTIR, and UV-Vis Spectroscopic Analysis

Year 2023, Volume: 5 Issue: 3, 324 - 332, 31.12.2024

Thennakoon Gamage Ramesh Ramawickrama G. Chamal Jalıya Iıllangasinghe Kalukumara M. S. C. Kumara Illangasınghe Kuruppu Arachchige D. D. Nimanthika Kuruppu

Abstract

Over 75% of the gem-bearing gravels in Sri Lanka belong to the corundum species of gemstones. Geuda is a low-quality form of corundum, and it is abundant in most gem-enriched regions in Sri Lanka. It can be heat treated to transfer into worthy blue sapphires where the value alters with the degree of colour. However, the degree of colour enhancement is uneven in Geuda stones even if the heat treatment is conducted under the same conditions. Thus, this study focuses on evaluating the effect of localization on the colour enhancement of the milky Geuda variety. Fifty Geuda samples were collected from five gem-bearing localities in Sri Lanka; Ratnapura, Ridiyagama, Okkampitiya, Bakamuna, and Katharagama. These samples were washed and sonicated with dil. HNO3 acid and facetted to have parallel smooth surfaces. Then all samples were observed and analyzed under a microscope, ED-XRF, FTIR, and UV-Vis spectroscopies and heated up to 1800 °C temperature for 60 minutes of the soaking period under reducing conditions. Then the analyses except XRF were repeated (before and after the heat treatment). Microscopic observations support visually identifying the degree of colour enhancement and inclusions changes. UV-Vis spectrums identified the cause of blue colour enhancement as the intervalence charge transfer (IVCT) developed by Fe2+ and Ti4+ ions, and the change in the degree of colour enhancement was also verified as an alteration in the degree of IVCT by the available ions. FTIR spectrum characterized the bond vibrational changes with the heat treatment, and FTIR was clarified as the best method to identify heat-treated blue sapphires since a characteristic peak was generated at 3310 cm-1 after treatment. Apart from Ti and Fe, V was also determined to affect blue colour enhancement by ED-XRF analysis. The optimum blue colour was enhanced in the ratios of 1: 3: 323 for Ti: Fe: Al. there were significant changes in the location and ternary diagrams are accompanied to record the highest Ti content in Ridiyagama and the highest Fe content in Bakamuna and Katharagama. Furthermore, corresponding unique elemental ratios for these localities were encountered.

Keywords

ED-XRF and FTIR analysis, Localization effect, Milky Geuda, Heat treatment of Geuda, Geuda identification

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