Gneissis is a common and widely distributed type of high-grade metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of ignbeous or sedimentary rocks. It can formed in several different ways. The most common path begins with shale, which is a sedimentary rock. Regional metamorphism can transform shale into slate, then phyllite, then schist, and finally into gneiss.