What is a Gneiss and how is it formed?
A gneiss is a coarse-grained metamorphic rock with a streak foliation due to parallel alignment of minerals, usually in bands of light and dark-colored minerals.
Gneiss’ typically form by regional metamorphism at convergent plate boundaries. It is a high-grade metamorphic rock in which mineral grains recrystallized under intense heat and pressure. This alteration increased the mineral grains’ size and segregated them into bands, a transformation that made the rock and its minerals more stable in their metamorphic environment.
Sources: https://mrdata.usgs.gov/geology/state/sgmc-lith.php?text=gneiss