Gneiss is a high-grade metamorphic rock characterized by distinct compositional and mineralogical banding, a feature known as foliation. This type of rock forms when pre-existing sedimentary or igneous rocks undergo intense heat and pressure deep within the Earth's crust. The intense metamorphic process causes the minerals within the rock to recrystallize and realign perpendicular to the greatest stress, resulting in the visible layering. Typical mineral compositions often include quartz, feldspar, and various micaceous minerals, which combine to create stripes of varying coloration and chemical makeup. The presence of these bands allows geologists to determine the metamorphic grade and the composition of the rock's original parent material.