Divalent iron, frequently substituting for magnesium, produces the dark green or black colors of many silicate minerals. Tremolite, a magnesium amphibole, is white, whereas actinolite, essentially the same mineral but with iron substituting for magnesium, is green. There is a continuous compositional range between the two minerals. Allochromatic Colors Pure quartz is clear, but can be any color at all due to allochromatic coloring agents. Purple quartz, or amethyst, owes its color to As testimony to how complex crystal-field interactions can be, chromium is an allochromatic coloring agent responsible for both the green of emerald and the red of ruby.
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.