metamorphic
Quartz-rich gneiss
Banded quartz–feldspar–mica gneiss
AI-generated identification · may be incorrectCoarse-grained, strongly foliated rock with white-gray quartz and feldspar bands plus dark biotite or amphibole. Quartz has Mohs hardness 7; feldspar 6, with a glassy to pearly luster and irregular cleavage; overall specific gravity is about 2.6–2.8.
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Physical properties
Coarse-grained, strongly foliated rock with white-gray quartz and feldspar bands plus dark biotite or amphibole. Quartz has Mohs hardness 7; feldspar 6, with a glassy to pearly luster and irregular cleavage; overall specific gravity is about 2.6–2.8.
Formation & geological history
Formed when preexisting granite or sedimentary rock underwent high-grade regional metamorphism, causing recrystallization and mineral segregation; commonly Precambrian to Paleozoic in age. The image’s weathered surface makes exact identification uncertain without a fresh break or testing.
Uses & applications
Used as crushed aggregate, road stone, building stone, countertops, paving, and decorative slabs; attractive pieces may be collected but are generally not gemstone material.
Geological facts
Its light-dark banding records intense pressure and deformation deep in the crust. Gneiss can resemble granite, but its aligned mineral bands distinguish it from massive, unfoliated granite.
Field identification & locations
Look for alternating pale quartz/feldspar and dark mica-rich layers, visible foliation, and a hard, granular texture; common in ancient continental shields and mountain belts worldwide. A streak, acid, and hardness test can help separate it from marble or quartzite.
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