Origin of the different color of ruby and emerald

The different color exhibited by ruby and emerald is a fundamental but still unsolved question. According to recent EXAFS measurements, such a difference can hardly be explained on the basis of a different average distance between Cr³⁺ and the six oxygen ligands. The puzzling difference in color between the two gemstones is shown in this work to arise essentially from the distinct electrostatic potential imposed by the rest of lattice ions upon the active electrons of the CrO₆^9- unit. Main effects are shown to come from the electric field generated in the neighborhood of the Cr³⁺ site in ruby which is absent in the case of emerald due to symmetry.