Effect of fusion mixture treatment on the surface of low grade natural ruby

Graphical abstract: The colour of low grade natural ruby is enhanced with fusion mixture treatment. Comparative optical absorption and photoluminesence properties of both untreated and treated ruby samples are studied. - Highlights: • Colour of the low grade natural ruby is improved with fusion mixture treatment. • Surface impurities are removed with fusion mixture. • Photoluminescence spectrum of ruby influenced by its Cr³⁺ concentration. • X-ray diffraction study confirms the presence of corundum phases in ruby samples. • Treated ruby looks brighter than untreated ruby due to variation in Cr³⁺ concentration. - Abstract: Improvement in aesthetic look of low grade natural ruby (gemstone) surface was clearly evident after fusion mixture treatment. Surface impurities of the gemstone were significantly reduced to give it a face lift. The processing consists of heat treatment (1000 °C) of the raw gemstone with fusion mixture (sodium and potassium carbonates), followed by hydrochloric acid digestion (90 °C) and ultrasonic cleaning.Both the untreated and the treated gemstone were characterized by X-ray diffraction, UV–vis spectroscopy (diffuse reflectance),photoluminescence and X-ray photoelectron spectroscopy. The paper consolidates the results of these studies and presents the effect of the typical chemical treatment (stated above) on the low grade natural ruby. While X-ray diffraction study identifies the occurrence of alumina phase in both the treated and the untreated gemstones, the UV–vis spectra exhibit strong characteristic absorption of Cr³⁺at 400 and 550 nm wavelength for the treated gemstone in contrast to weak absorption observed for the untreated gemstone at such wavelengths, thus showing the beneficial effect of fusion mixture treatment. Peaks observed for the gemstone (for both treated and untreated samples) in the excitation spectra of photoluminescence show a good correlation with observed UV–vis (diffuse reflectance) spectra. Photoluminescence emission spectra of the untreated gemstone show characteristic emission at 695 nm for Cr³⁺ ion (as in alumina matrix), but its emission intensity significantly reduces after fusion mixture treatment. It is found that the surface of the fusion mixture treated ruby gemstone looks much brighter than the corresponding untreated surface.