Abstract: A new type of synthetic sapphire with diffusion treatment has recently appeared on gem markets, which is treated from the flame fusion synthetic colourless sapphires by the diffusion with the compound of iron and titanium at super-high temperatures. The gemmological characteristics and diffusion mechanism of diffusion-treated synthetic sapphire are investigated in depth by using EDMA, FTIR, UV-VIS and so on. The results show that the main chemical compositions of infiltrated layer of the diffusion-treated synthetic sapphire consist of w(FeO_T)=1. 69%～1. 02% and w(TiO₂)=0. 57%～0. 39%, which are not well distributed on transection and the chemical compositional zoning occurs from the surface to the interior of the samples. The colouration of the diffusion-treated synthetic sapphire is mainly attributed to b band(573 to 578 nm) among the VIS absorption spectra, which results from the charge transfer of Fe²⁺/Ti⁴⁺. When the ratio of w(FeO_T)∶w(TiO₂) is less than 3 ∶1, it is suit able for keeping the charge transfer of Fe²⁺/Ti 4+ to form the blue infiltrated layer. A group of weak F TI R absorption bands at 3371, 3309, 3231cm^-1 are attributed to the v(OH) stretching vibration, and the weak absorption band at 3186cm^-1 is attributed to the v(NH) stretching vibration. The irregular net-like quenched cracks and etching hollows by the high temperature exit in the surface of some samples and a few of gas bubbles exit in the interior. The samples are uneven bluish white fluorescence under S W ultraviolet. The research indicates that the micro-diffusion transference of Fe²⁺ and Ti⁴⁺ ions in the diffusion treated synthetic sapphire is controlled by vacancy transference energy, vacancy diffusion coefficient and vacancy concentration gradient.