Gneiss: medium- to high-grade regional metamorphic rocks characterized by alternating granular and schistose layers of Quartz, Plagioclase, Potassium Feldspar, Biotite, Amphiboles, Muscovite, and other minerals. Gneisses are classified by their dominant mineral, such as Amphibole Gneiss or Garnet-bearing Gneiss. Orthogneisses derive from Granites or Diorites, while paragneisses originate from Sandstones, Conglomerates, and Clays. The schistose texture typically has a scale ranging from centimeters to millimeters.
