A MONOCHROMATIC beam of cathode rays was directed against a cleavage face of calcite at a grazing incidence ... [111] in the figure), and also many black and white lines (e.g. [130] in the ...

calcite can be white or colorless, but often contains impurities or trace elements. These can affect its color, giving it a range of hues, including blue, green, red, lavender, black, orange ...