metamorphic
Weathered gneiss cobble
Felsic gneiss, typically quartz–feldspar–mica bearing
AI-generated identification · may be incorrectMedium- to coarse-grained, pale gray/tan rock with dark biotite or amphibole patches and weak compositional banding; quartz has Mohs hardness 7, feldspar about 6, with vitreous to dull luster and no consistent cleavage as a whole.
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Physical properties
Medium- to coarse-grained, pale gray/tan rock with dark biotite or amphibole patches and weak compositional banding; quartz has Mohs hardness 7, feldspar about 6, with vitreous to dull luster and no consistent cleavage as a whole.
Formation & geological history
Formed when granite or related crustal rock underwent high-grade metamorphism involving intense heat and pressure, commonly during Precambrian to Paleozoic mountain building; the rounded shape reflects stream or glacial transport and weathering.
Uses & applications
Used as crushed aggregate, riprap, road stone, landscaping material, and occasionally dimension stone; this ordinary weathered cobble has little commercial or jewelry value.
Geological facts
Gneiss commonly preserves alternating light quartz-feldspar and dark mica-rich bands, distinguishing it from more uniform granite. The visible moss and surface staining are weathering, not mineral composition.
Field identification & locations
Look for mineral segregation or banding, interlocking grains, and abundant quartz and feldspar; distinguish it from granite by its foliation and from schist by its lower mica content. Common in ancient continental shields and glacial or river gravels.
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