Gneiss << nys >> is a banded, coarse-grained rock. In most forms of gneiss, the banding results from the arrangement of dark- and light-colored minerals into alternating layers. In some, it is produced by the parallel alignment of platelike or needle-shaped crystals. Unlike schist and various other banded rocks, gneiss does not readily break along its layers.
Gneiss is a common variety of metamorphic rock, one of the major kinds of rocks. It is formed during the regional metamorphism of the two other main types of rocks, igneous and sedimentary. In this process, heat and pressure alter the original rock deep within Earth’s crust. Gneiss that has been formed from igneous rock is called orthogneiss. It consists of feldspar, quartz, and ferromagnesian silicates (see Silicate). Gneiss that originates from sedimentary rock is called paragneiss. Paragneiss consists of feldspar, quartz, and such other minerals as muscovite and kyanite.
