What causes the blue colour of sapphire

Sapphires are naturally occurring gem stones, and are transparent to translucent. We know from ancient records that they have always been highly prized. Natural sapphires are found in many igneous rocks, especially syenites and pegmatites. Alternatively, synthetic sapphires may be made by doping aluminium oxide AI2O3 with a chro-mophore. 

But why is sapphire blue Alumina is colourless, yet neither iron nor titanium commonly form blue compounds. The colour arises from a charge-transfer type of 

The frequency of the light absorbed to effect the photo-excitation is in the red and near-infrared parts of the visible region, so the complementary colour seen is blue. This explains why sapphire is blue. It is again a charge-transfer excitation, but not of the mixed-valence type. The optical band formed is intense, so a strong colour is seen even though the concentrations of iron and titanium are minuscule. 

The system relaxes back to the starting materials (the reverse of Equation (9.18)) within a fraction of a second (probably 10-15 s). 

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