Abstract: The controversial question whether the colour of Madagascar orange sapphire is caused by the diffusion treatment, by the high temperature treatment, or by the irradiation colouring can be answered by the application of such testing methods as ICP-MS, EPMA, ESR, UV and the comprehensive correlation research on this type of orange sapphire. As shown in the research results, the chemical composition of the orange sapphire is relatively rich in Cr and poor in Fe, Ti, the chemical mixture ratios of the transitional metal ions are relatively homogenous from the exterior to the interior, and the abundance of Be is locally concentrated on the surface of a few samples. The Fe-O^- impure ion center makes the absorption band move to the blue violet (454 nm), which superimposes on the absorption band resultled from the Cr ^{3+ 4}A ₂(⁴F)→ ⁴T ₁(⁴F)ion spins, and then combines with the residual energy from the Cr ³⁺ ion absorption into the hue of the orange sapphire. The initial investigation proves that the foreign Be ²⁺ is not included in the colouring ions. It is not related to the colouring of the sapphire, and serves as a kind of activator or channel diffusion. The major external conditions for the orange-yellow colouring are the high temperature, increase in oxygen and Be activator, while the major internal conditions are Fe-O^- impure ion center, Cr ³⁺ and Fe ³⁺ ions. Furthermore, the thickness of the orange yellow layer is constrained by the temperature and time interval of the high temperature heat treatment, and the valence state and concentration of the replacing impure ions (Fe, Cr, Ti) and the density of the defect in the crystal lattice.