Abstract: Guilin hydrothermal synthetic ruby and sapphire are a new product after the marketing of Tairus hydrothermal synthetic ruby and sapphire. Studies reveal the following characters: Guilin hydrothermal synthetic ruby has the colour of orange and the absorption spectrum which is different from that of the natural ruby and orange-red sapphire(Pad), with its colour being caused by such transitional ions as Cr ³⁺, Mn ³⁺ and Ni ³⁺; light yellow synthetic sapphire has its colour caused by Ni ³⁺ and its absorption spectrum differs from that of the natural yellow sapphire; the most universal characteristics for identification are the residual of the seed crystal, dust-like inclusions distributing as planar, and the lack of apparent growth lamination and hydrous infrared absorption spectrum.