Abstract: The colour holds considerable importance in the both classification and valuation of sapphire. In recent years, some gemstone markets have been troubled by many fancy sapphires, even those backed by authoritative certification, that have experienced varying degrees of fading or colour change after a period of displaying or wearing. These unstable colour sapphires exposed to sunlight or ultraviolet light, show reversible colour alterations—the phenomenon ascribed to the photochromism. For many years, the photochromic effect in sapphires has attracted the attention of gemologists. Previous studies indicate that the photochromic phenomena are common in a range of sapphires, including colourless, light blue, yellow, pink, Padparadscha varieties and so on. Meanwhile, whether natural or synthetic, and heated or irradiated, there may be photochromic phenomena in these gemstones and its primary feature of the colour change is the fluctuation in yellow tone. In order to effectively identify the photochromic sapphires, many well-known authoritative testing institutions have carried out fading tests. However, the underlying colour mechanism of photochromic sapphire is still unclear.In this paper, 75 natural yellow sapphire samples fromv Sri Lanka, Madagascar and unknown origin were studied for photochromic experiments. The yellow sapphire samples were coloured and faded by using the 254 nm ultraviolet light source and D65 light source (simulated sunlight) respectively. By UV-Visible absorption spectrum, emission spectrum and excitation spectrum of the sapphires, the test results show a the covariation between the colour instability of the yellow sapphire and its instability of orange fluorescence.It can be inferred that the defect that releases orange fluorescence plays a crucial role in the photochromic effect of yellow sapphires. This is presumed to be associated with electron transfer between defects-specifically, Mg²⁺-related defects involving trapped holes and F-centers during the photochromic process. Consequently, the sapphire sample has been ground into powder and its colour instability was analyzed by EPR spectrum. The results show that notable alterations are detected in the region at g≈2.0 before and after the colour change, which is indicative of the presence of trapped-hole and F-centers. At the same time, synchrotron X-ray absorption spectra of Mg elements in samples were also analyzed. The shifts in the pre-edge and edge of the XANES spectra suggest octahedral distortion, which supports the electron transfer in Mg²⁺ related defects during the photochromism process. Additionally, alterations in the fluorescence of Cr during the photochromic process have also been noted. The PL spectra of the sapphire sample pre- and post-colour change have been tested, with observations showing that the pattern of orange fluorescence exhibited an inverse relationship to that of Cr fluorescence. This may be due to Cr combining with trapped-hole within the corundum structure. To verify that the process don't introduce new defect levels, the fluorescence lifetime of Cr has been measured. The results indicated that the fluorescence lifetime did not change, confirming that no new energy levels were involved in the emission of Cr, except for the trapped-hole and F-centers during the photochromic process. Our findings indicate that the essence of the photochromic process of yellow sapphire is the electron transfer between trapped-hole and F-centers, which could also influence the electron transitions within impurity elements like Cr.