"A crystal of containing a few hundredths of one percent of is colorless. If, instead it contains a similar amount of iron, a very pale yellow may be seen. If both impurities are present together, however, the result is a magnificent deep blue color, that of blue ... The process at work is ', the motion of an electron from one to another produced by the absorption of light energy; this results in a temporary change in valence state of both s. Such a mechanism is the cause of the blue sapphire and the black and dark colors of many transition metal oxides such as iron oxide ' Fe3O4. The mechanism is sometimes also called cooperative charge transfer. Somewhat analogous charge transfer processes also occur in ligand field situations; this can lead to color even if there are no unpaired electrons to cause the color by... transition mechanisms... [I]t is possible to describe many of the donor-acceptor dyes... as involving charge transfer, although... not commonly used."