The origins of color in minerals Kunr Nlsseu Bell Laboratories

Four formalisms are outl ined. Crystal f ield theory explains the color as well as the f luorescence in transit ion-metal-containing minerals such as azurite and ruby. The trap concept, as part of crystal f ield theory, explains the varying stabi l i ty of electron and hole color centers with respect to l ight 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 nonmetalinvolving colors in lazuri te, graphite and organical ly colored minerals. Band theory explains the colors of metal l ic minerals; the color range black-red-orangeyel low-colorless in minerals such as galena, prousti te, greenockite, diamond, as well as the impurity-caused yel low and blue colors in diamond. Lastly, there are the well-known pseudochromatic colors explained by physical optics involving dispersion, scattering, interference,