Gneiss

Hardness about 6–7 Mohs; gray, tan, and rusty brown with coarse, wavy light–dark banding; generally nonmetallic, granular luster, and no prominent cleavage as a whole. Individual feldspar and mica grains may show cleavage; density is commonly 2.6–2.8 g/cm³.

A high-grade metamorphic rock formed when granite or related sedimentary rocks recrystallize under intense heat and pressure, commonly during Precambrian to Paleozoic mountain-building events. The visible banding results from mineral segregation during deformation and partial melting.

Used as dimension stone, landscaping and riprap, road aggregate, and building facing; attractive polished pieces may be used decoratively. This coarse field specimen has modest commercial value.

Gneiss commonly preserves evidence of deep crustal conditions and tectonic compression. Its alternating quartz–feldspar-rich and mica-rich bands distinguish it from more uniformly textured granite.

Identify it by examining the strong compositional banding, coarse interlocking grains, and resistance to scratching with a steel blade; quartz-rich grains may scratch glass. Common worldwide in ancient continental shields and mountain belts, including Scandinavia, Canada, the northeastern United States, India, and Australia.