Polarised light micrograph of gneiss from Upper Kesselalm, Proveis, Deutschnonsberg, South Tyrol, Italy. Gneiss is a common and widely distributed metamorphic rock. It is formed by high temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. 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: x4 when printed at 15 centimetres wide.