Abstract: The fading and discoloration of sapphires have emerged in recent years, bringing panics and confusions to the jewellers, consumers and gemmologists. To study the behavior and mechanism of tenebrescence in sapphire, multi light source irradiation tests and in situ photography were conducted on the collected specimens involving non-treated natural sapphire, heat treated natural sapphire and synthetic sapphire. Ultraviolet-visible (UV-Vis) spectra, fluorescence spectra, electron paramagnetic resonance (EPR) spectra and infrared spectra of the sapphire specimens were tested and analysed. With photography and UV-Vis spectroscopy, the photochromic impacts by various light sources were discussed, including 370 nm UV LED, DiamondView^TM, high pressure mercury lamp, low pressure mercury lamp, and Skyray EDX3600H EDXRF. The results showed that the tenebrescent effect is quite common in fancy colour sapphires from Sri Lanka and Madagascar with a light tone. After exposure to UVA, UVC or X-rays, new colour centers are induced within the sapphires and result in abroad, blue-green region centered absorption band which extends from the ultraviolet region to the red region around 750 nm, giving the sapphires an extra brown (orange) tone and lowering their brightness. Irradiation with white light can slowly bleach the broad absorption band and recover the initial colour. Hence a basic model of tenebrescent process in sapphire was proposed: UV/X-rays irradiation shifts sapphires from bleached (B) state to coloured (C) state, while white light irradiation reverses the reaction. When exposed to a mixture of both white light and UV/X-rays, an equilibrium position between B state and C state is to be achieved. Since the ratio of white light and ultraviolet light in the luminous environment dominates the colour performance of sapphire, geographical movement as well as indoor storage and display of sapphire often lead to alteration in the colour. In view of the predominant presence of UVA in the ultraviolet region of sunlight spectrum at earths surface, UVA-induced tenebrescent effect is suggested to be considered as a natural colouration, while UVC and X-rays should be classified as artificial irradiations. Gemmological indication of UVA tenebrescent coloration is found to be the yellow-orange fluorescence under UVA irradiation, which can be recognized by visual observation or fluorescence spectrum analysis. In addition, a rare infrared "tenebrescent" phenomenon is accidentally discovered in Verneuil synthetic light pink sapphires.