Gneiss (pronounced (nice) is a foliated metamorphic rock characterized by distinct banding or layering, caused by the segregation of light- and dark-colored minerals. Appearance * Shows alternating light and dark bands * Light bands:mostly ,quartz and feldspar* * Dark bands: contain biotite, hornblende, or pyroxene * Often coarse-grained and very hard. Formation (Metamorphism) Gneiss forms through: 1. Regional metamorphism: deep burial and compression over large areas, such as mountain belts. 2.