metamorphic
Gneiss (probable, weathered fragment)
Foliated metamorphic rock composed mainly of quartz, feldspar, and mica; composition may be granitic or migmatitic.
AI-generated identification · may be incorrectTypically hard (about 6–7 Mohs), coarse-grained, with alternating light and dark mineral bands, though this specimen is strongly weathered and irregular. Luster ranges from dull to glassy on quartz and feldspar surfaces; no prominent cleavage as a whole rock.
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Physical properties
Typically hard (about 6–7 Mohs), coarse-grained, with alternating light and dark mineral bands, though this specimen is strongly weathered and irregular. Luster ranges from dull to glassy on quartz and feldspar surfaces; no prominent cleavage as a whole rock.
Formation & geological history
Formed when granite or other silicate rock underwent high-grade metamorphism involving intense heat and pressure, commonly during mountain building. Precise age cannot be determined from the image; gneiss ranges from Archean to younger geologic ages.
Uses & applications
Used as crushed aggregate, dimension stone, landscaping rock, and occasionally decorative facing stone; attractive pieces may be collected but are rarely high-value.
Geological facts
The banded texture reflects mineral segregation during metamorphism, with quartz-feldspar-rich layers contrasting with biotite- or amphibole-rich layers. The surrounding surface appears to be asphalt or concrete aggregate, not part of the specimen.
Field identification & locations
Identify by coarse interlocking grains and compositional banding; fresh broken surfaces should show quartz, feldspar, and dark mica. Common in ancient continental shields and metamorphic mountain belts worldwide.
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