Abstract
Four formalisms are outlined. Crystal field theory explains the color as well as the fluorescence in transition-metal-containing minerals such as azurite and ruby. The trap concept, as part of crystal field theory, explains the varying stability of electron and hole color centers with respect to light or heat bleaching, as well as phenomena such as thermoluminescence. The molecular orbital formalism explains the color of charge transfer minerals such as blue sapphire and crocoite involving metals, as well as the nonmetal-involving colors in lazurite, graphite and organically colored minerals.
Band theory explains the colors of metallic minerals; the color range black-red-orange-yellow-colorless in minerals such as galena, proustite, greenockite, diamond, as well as the impurity-caused yellow and blue colors in diamond. Lastly, there are the well-known pseudochromatic colors explained by physical optics and diffraction.
