metamorphic
Banded gneiss
Gneiss, commonly quartz–feldspar–mica gneiss
AI-generated identification · may be incorrectTypically coarse-grained, with alternating light quartz/feldspar bands and dark biotite or amphibole; the pronounced streaked texture is gneissic foliation. Hardness is about 6–7 Mohs, nonmetallic to vitreous luster, and weak cleavage inherited from feldspar and mica.
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Physical properties
Typically coarse-grained, with alternating light quartz/feldspar bands and dark biotite or amphibole; the pronounced streaked texture is gneissic foliation. Hardness is about 6–7 Mohs, nonmetallic to vitreous luster, and weak cleavage inherited from feldspar and mica.
Formation & geological history
Formed by high-grade metamorphism of granite or sedimentary protoliths under intense heat and pressure, commonly during Precambrian to Paleozoic mountain-building events. The exposed boulder may have been shaped by glacial transport and abrasion.
Uses & applications
Used as crushed aggregate, road stone, riprap, building stone, paving, and landscaping material; attractive blocks may be polished for countertops or monuments. Ordinary field specimens have modest collector value.
Geological facts
Gneiss is identified by compositional banding rather than the thin, closely spaced foliation typical of schist. The strongly corrugated surface likely reflects differential weathering or glacial abrasion along mineral bands.
Field identification & locations
Common in ancient continental shields and metamorphic mountain belts, including the Canadian Shield, Scandinavia, New England, and the Appalachian region. Look for alternating light and dark mineral bands, coarse crystals, and a hard, granitic feel.
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