Some minerals do not show transparency in thin sections. They are opaque. Opaque minerals can not be observed with a transmission light microscope. In this case a reflected light microscope has to be used instead of a transmission light microscope. The degree of reflectivity of a crystal varies from approximately 10 to 100 %. The relative reflectivity is defined by the ratio between the light reflected by the mineral and the incident light and is given in percent. The degree of reflectivity has to be compared with the amount of reflectivity of a known material. The visual impression of reflectivity might be influenced by the degree of reflectivity of a neigboring crystal. A moderately reflecting crystal might be observed as pale if it is surrounded by strongly reflecting crystals. The degree of reflectivity varies with the quality of polishing, therefore, an equally well polishing of the specimen is necessary for determination of reflectivity.