Gneiss

Typically medium-to-coarse grained, with alternating light quartz/feldspar and dark biotite or amphibole bands; hardness about 6–7, vitreous to dull luster, and weak foliation rather than obvious cleavage. The photographed specimen appears strongly banded and granular, though exact identification is limited by the image.

Forms when granite or other continental crustal rocks undergo high-grade regional metamorphism under intense heat and pressure, commonly during mountain building. Many gneisses are Precambrian to Paleozoic, but their age depends on the source rock and metamorphic event.

Used as dimension stone, paving, wall stone, aggregate, and decorative slabs; attractive pieces are collected but are generally low-value.

Its compositional banding reflects mineral segregation during metamorphism, not sedimentary layering. Dark bands commonly contain biotite, hornblende, or pyroxene, while light bands contain quartz and feldspar.

Look for coarse interlocking crystals and alternating light-dark mineral bands, distinguishing it from finer-grained schist or massive granite. Common worldwide in ancient continental shields and metamorphic mountain belts.