Trace impurities of ferrous iron substitute for the aluminum and give it the bluish hue that gives the mineral its characteristic field appearance and name. Charge transfer occurs between iron and oxygen. Chromates derive their intense hues from charge transfers between chromium and oxygen. Iron is far and away the dominant coloring agent in rocks and soils. All the colors in this picture are due to iron.
Divalent iron makes silicate minerals dark green or black.
Trivalent iron participates in charge transfers that are strongest in the ultraviolet, but somewhat effective in the visible range. In the last century, we have learned how to synthesize compounds with molecular orbital colors. Some chemical reactions among organic chemicals give off light, like in this firefly. We have recently learned to synthesize chemiluminescence. In the mid 19th century, we began to learn how to create molecular orbital colors ourselves, though we learned how to make the colors before we really understood the mechanism.