Beryllium Heat-Treated Blue Sapphire

Since 2000, corundum heat-treated by beryllium (Be) has been successfully experimented to lighten the over-dark blue basaltic-related sapphire. In early 2006, advancements in heat treatment techniques in Chanthaburi revealed the ability to darken metamorphic-related blue sapphires using Be treatment, contrary to the previous lightening effect. The role of Be in corundum coloration obviously remains not well understood. Therefore, an experiment involving heat treatment was conducted to study changes in terms of physical, chemical, gemological, and spectroscopic properties of the blue sapphire after beryllium heat treatment, which has never been done before. Natural sapphires from Sri Lanka, varying in color and transparency, were subjected to Be heat treatment across a range of temperatures and in different types of furnaces (e.g. LPG gas, electric, and fuel oil), under both oxidizing and reducing atmospheres. The samples were studied after each heating step for gemological and spectroscopic properties using UV-Vis-NIR and infrared spectroscopy, as well as chemical composition using LA-ICP-MS. The combination of color appearance, absorption spectra, and chemical data suggests the role of beryllium in sapphire coloration. Be-trapped hole yellow color centers formed during oxidation heating can be deactivated through reduction heating. The resulting color is determined by the Mg to Ti ratio, with no influence from Be. However, most beryllium-treated blue sapphires had an Mg/Ti ratio greater than 1. The presence of higher energy level elements, such as Si, may facilitate the formation of blue color by allowing remaining titanium to pair with iron. Therefore, the blue coloration is mainly caused by strong broad absorption bands of Fe2+/Ti4+ IVCT mechanism without Fe2+/Fe3+ IVCT. The study highlights the complex interplay of elements within the crystal lattice and contributes to our understanding of color development mechanisms in beryllium-treated sapphires. HIGHLIGHTSA new method for identifying beryllium-treated blue sapphires based on combination of changes in physical, chemical, gemological, and spectroscopic properties after heat treatment is proposed in order to provide new evidence of indication of heating. In order to better understand the cause of color in beryllium-treated blue sapphire from Sri Lanka, a new method of explanation the role of Be and Si is proposed. GRAPHICAL ABSTRACT