Polarised light micrograph of paragneiss (biotite-quartz-plagioclase gneiss) from Hechtsberg, Black Forest, Germany. Gneiss is a common and widely distributed type of metamorphic rock formed by high temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Paragneiss is gneiss derived from sedimentary rock, such as sandstone). Gneiss nearly always has a banded texture characterised by alternating darker and lighter bands, without a distinct foliation. The darker bands contain more mafic minerals, such as magnesium and iron. The lighter bands contain relatively more felsic minerals, such as silicon, oxygen, aluminium, sodium, and potassium. Magnification: x30 when printed at 15 centimetres wide.