metamorphic
Banded gneiss
Gneiss, commonly quartz–feldspar–biotite gneiss
AI-generated identification · may be incorrectTypically medium-grained, with alternating dark biotite/amphibole and light quartz–feldspar bands; hardness about 6–7, vitreous to dull luster, and weak or no cleavage as a whole. The specimen shows pronounced foliation and compositional banding, though its blurred image limits certainty.
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Physical properties
Typically medium-grained, with alternating dark biotite/amphibole and light quartz–feldspar bands; hardness about 6–7, vitreous to dull luster, and weak or no cleavage as a whole. The specimen shows pronounced foliation and compositional banding, though its blurred image limits certainty.
Formation & geological history
Formed when preexisting igneous or sedimentary rock underwent intense high-grade metamorphism, commonly during continental mountain-building. Individual gneisses may range from Precambrian to younger ages.
Uses & applications
Used as dimension stone, facing stone, countertops, paving, landscaping rock, and aggregate; attractive banded pieces are collected but usually have modest value.
Geological facts
Gneissic banding results from mineral segregation and deformation under high temperature and pressure, not simple layering during deposition. It commonly represents metamorphic grades higher than schist.
Field identification & locations
Identify it by alternating light and dark mineral bands, a hard crystalline texture, and foliation; unlike granite, the minerals are arranged into streaks rather than a uniform mosaic. Common in ancient continental shields and metamorphic mountain belts.
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