Corundum Visible Spectra (generally 350 - 1200 nm)

Corundum occurs in several colored varieties. Ruby is red because of its Cr3+ content. Yellow sapphire owes its color to Fe3+. Blue sapphire derives its color from the Fe2+ - Ti4+ and the Fe2+ - Fe3+ intervalance charge transfer. Green sapphires contain a mixture of the blue and green colors. 

Corundum Infrared Spectra (generally 4000 - 3000 cm-1)