Natural tanzanites usually show strongly trichroic coloration from violet to blue, and brown colors in different directions. However, this characteristic is easily changed to violet-blue dichroism after heat treatment. Moreover, the cause of color modification after heating is still controversial. A few researchers have previously suggested that trace amounts of either vanadium or titanium substituted in aluminum site should be the main determinant of color after the heat treatment. Alteration of either V3+ to V4+ or Ti3+ to Ti4+ may relate to light absorption around 450–460 nm, which is the main cause. UV/vis/NIR absorption spectroscopy and X-ray absorption spectroscopy (XAS), a utility of synchrotron radiation, were applied for this experiment. As a result, the violet-blue absorption band (centered around 450–460 nm) as well as green absorption band (centered around 520 nm) were obviously decreased along the c-axis after heating, and XAS analysis indicated the increasing of the oxidation state of vanadium. This result was well supported by the chemical composition of samples. Consequently, vanadium was strongly suggested as the significant coloring agent in tanzanite after heat treatment.

Natural spinel (MgAl2O44) usually contains some minor and trace elements (e.g., Cr, Co, Fe, V) that may cause various hues. The ratios of these chromophores directly affect the color composition. The red color in spinel is attributed to the combination of significant Cr and V. Magenta and purple to blue and green colors in spinels are affected by the significant Fe concentration, whereas orange color in spinel shows the contribution of significant V content compared to Cr and Fe. After the heating experiment, advanced gemological investigation reveals some noteworthy characteristic features. X-ray absorption spectroscopy (XAS) indicates a greater change in oxidation state, as well as disordering of Fe and V. Broadening of the dominant peak at around 406 cm−1 with occurrences of additional small peaks at around 715–719 cm−1 in Raman spectra, as well as broadening of the 685 nm (R-line) and poorly defined structure of additional peaks (N-lines) in photoluminescence spectra should be significant indicators of spinel undergone heat treatment.