Banded gneiss

Typically medium-coarse grained with alternating light quartz/feldspar and dark biotite or amphibole bands; vitreous to pearly luster, Mohs hardness about 6–7, poor cleavage overall but foliation and irregular fracture. The specimen appears dark, foliated, and weathered, though exact mineral identification is limited by the image.

Formed by high-grade regional metamorphism that recrystallized preexisting igneous or sedimentary rock, commonly during Precambrian–Paleozoic mountain building. Its banding developed from mineral segregation under intense heat and pressure.

Used as dimension stone, paving, landscaping rock, aggregate, and decorative slabs; attractive pieces are collected but generally have modest value.

Gneiss is identified by compositional banding rather than thin, evenly spaced cleavage like slate. Some gneisses preserve evidence of ancient continental crust and deep crustal conditions.

Common in continental shields and deeply eroded mountain belts, including Scandinavia, Canada, the northeastern United States, India, and Australia. Check for alternating mineral bands, visible interlocking grains, and a hard, nonporous surface; a streak, magnet test, and fresh broken face improve identification.