Reddish-Brown Zircons of Sri Lanka: A Detailed Study and Development of a New Technique for Yellow to Golden Yellow Colour Enhancement

Sri Lanka is famous for various types of gem minerals. Among these minerals, gem-quality zircon is found in both primary and secondary deposits. The main objective of the research was to study the physical, chemical, and spectroscopic features of reddish-brown zircons (RBZ) of Sri Lanka and to develop a methodology of heat treatment for RBZ of Sri Lanka for colour enhancement. Five randomly selected RBZ samples were selected for this study. All samples were translucent, highly fractured, sub-adamantine, and with euhedral to subhedral crystal form. Samples were analysed with EDXRF, UV-Vis Spectrophotometer, and FTIR methods. The UV-Vis absorption spectrum indicates the cause of colour is due to structural defect colour center by radiation damage from radioactive elements such as U and Th. FTIR spectroscopy indicates that the RBZ of Sri Lanka has gone through a metamictization process.  Further FWHM of Raman spectroscopy confirms that RBZ of Sri Lanka belongs to the Intermediate-type. This is also confirmed by the specific gravity of the stones.  And more importantly, they were not detectably radioactive which encourages embedding into jewellery. Being the most abundant chemical substitution is Hf, while all the RBZ samples contained a low amount of REE and radioactive elements. Additionally, the ratio of Th: U was consistent with a magmatic origin. Hence, our results confirm that heat treatment under either oxidizing or reducing conditions in an electric furnace produces a yellow to golden yellow colour at around 500- 600°C with a soaking period of fewer than 90 minutes.