gems, spinel, Vietnam, Luc Yen, enhancements

In this study, 15 pink-to-red spinel samples from Luc Yen, Vietnam, were subjected to heat treatment at 850°C and 1000°C under oxidising conditions to investigate the causes of their colour modification. Heating to 850°C slightly reduced the yellow and blue colour modifiers, yielding a more desirable pink-to-red hue. Treatment at 1000°C further decreased these modifiers but also resulted in a darker, greyer appearance. UV-Vis and FTIR analyses showed that partial structural disordering occurred in the heated spinels, potentially enabling the reduction of yellow and blue hues. A shift to higher wavelengths of the ~538 nm Cr³⁺±V³⁺ absorption band in the UV-Vis spectrum, along with the absence of a 575 cm⁻¹ FTIR band, were identified as indicators of heat treatment. XANES spectroscopy revealed that some Fe²⁺ oxidised to Fe³⁺, likely causing the reduced blue colouration, while the valence state of vanadium remained unchanged.