short version this article by Kurt Nassau Kurt Nassau Kurt Nassau this article by http://www.minsocam.org/msa/collectors_corner/arc/color.htm Kurt Nassau American Mineralogist: The origins of color in minerals this article by this article by this article by http://www.minsocam.org/msa/collectors_corner/arc/color.htm a few boron atoms per million this article by Kurt Nassau this article by Kurt Nassau Kurt Nassau Kurt Nassau this article by this article by Kurt Nassau