absorption spectra and polarized absorption spectra of dark blue sapphire from Changle, Shandong, China, were analyzed. The wide

intervalence charge transfer absorption band centered at 869 nm in the near infrared region of Fe2+–Fe3+ ion pairs was examined. The

colorizing role of iron and titanium in dark blue sapphire was discussed. Based on the optical absorption spectra of dark blue sapphire,

Fe2+–Fe3+ ion pair can also render a corundum gemstone blue. In addition to Fe2+–Ti4+ ion pair, Fe2+–Fe3+ ion pair is the second color

mechanism of natural blue sapphire.