Depending on the crystallographic orientation minerals are able to absorb different ranges of wavelengths. This ability is called Pleochroism. If illuminated with unpolarised white light some minerals appear in different colours depending on the direction in which they are observed. If polarised light is used for illumination instead of natural unpolarised light another effect is added: the plane of polarisation of the light waves. Depending on the vibration direction of the mineral green light might be absorbed (therefore the mineral appears in red color). If this mineral is turned by 90 ° orange light might be absorbed and the mineral appears in blue color. For observation and analysis of pleochroism a polariser in the lightpath of the microscope is necessary. An analyser is not used. If the mineral can be rotated the polariser has to be in a fixed position.