Short answers, pulled from the story.
Gneiss is a common, widely distributed metamorphic rock formed by high-temperature and high-pressure processes acting on igneous or sedimentary rock. It forms under pressures from 2 to 15 kbar, sometimes more, and temperatures over 300 degrees Celsius, or 572 Fahrenheit.
Gneiss shows gneissic banding, alternating darker and lighter layers. The darker bands hold more mafic minerals rich in magnesium and iron, while the lighter bands hold felsic minerals such as feldspar and quartz. The bands form at high temperature when the rock is compressed more strongly in one direction, developing perpendicular to the shortening direction.
Orthogneiss is a gneiss derived from an igneous rock, while paragneiss is a gneiss derived from a sedimentary rock. Neither name depends on the rock's current composition, only its original protolith.
Augen gneiss is a gneiss formed from metamorphosed granite, named from the German Augen, meaning eyes. It contains elliptic or lenticular grains called porphyroclasts, usually feldspar, surrounded by finer grained material that deforms around them to create the eye-like texture.
The Acasta Gneiss in the Northwest Territories of Canada is among the most ancient intact crustal fragments on Earth, metamorphosed 3.58 to 4.031 billion years ago. It lies on an island about 300 kilometres north of Yellowknife.
Gneiss is used as a building material, including Facoidal gneiss used extensively in Rio de Janeiro. It is also used as construction aggregate for asphalt pavement.
Migmatite is a gneiss made of two or more distinct rock types, often interpreted as rock that has been partially melted. It includes a mesosome resembling ordinary gneiss, a silica-rich leucosome resembling intrusive rock like pegmatite, aplite, or granite, and sometimes a mafic melanosome.