Banded gneiss

Typically medium-to-coarse grained, with alternating light quartz/feldspar and dark biotite or amphibole bands; vitreous to pearly luster, Mohs hardness about 6–7, weak or absent cleavage, and variable specific gravity around 2.6–2.9. The specimen shows strongly folded, gray-green compositional banding and a weathered surface.

Formed when preexisting igneous or sedimentary rock underwent high-grade regional metamorphism, commonly during mountain building; individual gneiss bodies may range from Precambrian to younger ages. Exact age cannot be determined from the photograph.

Used as dimension stone, paving, wall cladding, aggregate, and landscaping rock; attractive polished pieces are collected but are usually inexpensive.

Its light-and-dark bands record mineral segregation and intense deformation under high temperature and pressure, often deep within continental crust.

Identify it by alternating mineral bands, coarse grains, and foliation; unlike layered sedimentary rock, bands are commonly tightly folded and crystalline. Common worldwide in ancient continental shields and metamorphic mountain belts.