Abstract: A variety of the diffused topaz is discovered in gem markets recently. This kind of stone is characterized by the absence of the colour in the internal and the concentration of blue green colour and brown yellow green colour speckled appearance on the surface (thickness less than 5μm). The reasearch indicates that the diffused topaz is coloured mainly by the permeance of Co ²⁺, and the crisscross pile silice texture associated with crystal defects is developed commonly in the permeable colour layer. The absorption within the visible light zone of 597, 632, 461nm is resulted from the transition of. ⁴T_1g (⁴F) → ⁴T_1g (⁴P)、 ⁴T_1g → ⁴A_2g 、 ⁴T_1g → ²T_1g in Co²⁺. It has been verified that the temperature of diffusion and crystal defects on the surface and concentration gradient of Co²⁺ are the main causes for the colouration of the diffused topaz. It can be concluded that the LRM is an important tool for identifying the diffused topaz.