Many colors are due to interactions between atoms. Charge-transfer colors result when photons transfer an electron from one atom to another. Molecular orbitals, common in organic materials, occur when electrons are bound collectively to entire molecules rather than single atoms. Kyanite is aluminum silicate, and ought to be colorless. 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.