Gneiss is a type of metamorphic rock, meaning it changed over time from a different type of rock to its current state. It has coarse bands of colour that look like stripes of varying thickness. This is called "foliation" and is caused by the minerals that make up the rock. These minerals moved in a plane as the rock changed, leaving the stripes. Gneiss by its definition cannot be unfoliated. The smoothness that characterises unfoliated rocks means that it is not gneiss.
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Formation of Gneiss
There are three types of rocks. Igneous rocks are formed from cooled lava. Sedimentary rocks are made from hardened dirt and sediment, hence the name. Metamorphic rock is rock that undergoes a change or metamorphosis and becomes a different type. The metamorphosis is caused by extreme pressure from the earth's crust or from heat -- 200 to 600 degrees Celsius (392 to44.4 degrees Celsius) after which it boils, according to the Physical Geography website. Gneiss is formed by metamorphosis from any of these types of rock. For example, sediment on the ground eventually becomes shale, a sedimentary rock. Shale can turn into gneiss, as can granite and basalt, both of which are igneous. Metamorphic rocks can continue changing, or can metamorphose again, and become gneiss.
Foliation refers to the look of the rock. During recrystalization when rocks metamorphose, the minerals that make up the rock will move in the path of least resistance, leaving layers of different minerals. This leaves bands of lighter and darker colours, since the minerals are often different colours. These bands are what make a rock foliated. Unfoliated rocks are those without bands of minerals, and may be one colour or mottled instead.
Gneiss has a texture to it because of the bands of rocks it is comprised of. The minerals do not recrystalize perfectly smoothly, and leave uneven bands and coarse-looking lumps. This coarseness in foliated rocks is called gneissic texture, and rocks with gneissic texture are then classified as gneiss. The rock itself may be smooth, either through wear or cutting, but the pattern of the bands are rough looking.
Minerals in Gneiss
Gneiss may be made up of a variety of minerals. For example, the rock's light-coloured bands are most often quartz and feldspar. Mica is darker, appearing as blackish or dark stripes. Other minerals may also be part of a gneiss rock. Windows to the Universe describes gneiss as looking like fudge ripple ice cream because of these mineral stripes.
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