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The difference in colour between emerald (Be3Si6Al2O18:Cr3+, green) and theCr3+-doped spinel MgAl2O4 (red) is striking, considering that in both systemscolour is due to CrO69- complexes with D3 symmetry and the measured Cr3+-O2-distance is practically the same (1.98 and 1.97 angstroms, respectively). Bymeans of density functional calculations it is shown that this surprisingdifference can reasonably be explained once the electric field, ER, which alllattice ions lying outside the CrO69- complex exert on localized electrons, istaken into consideration. The origin of the different shape of ER in the twohost lattices is analysed in detail. It is shown that ER raises (decreases) the2p(O) levels for Be3Si6Al2O18:Cr3+ (MgAl2O4:Cr3+) along the trigonal axis thusfavouring a decrease (increase) of 10Dq. The present work demonstrates the keyrole played by ER (not considered in the traditional ligand field theory) forunderstanding the differences exhibited by the same complex embedded in hostlattices which do not have the same crystal structure. Some remarks on thecolour of Cr2O3 pure compound are also reported.

MgAl2O4:Cr3+ and emerald display a different colour but the local symmetry is the same: Microscopic origin