metamorphic
Weathered granitic gneiss (probable)
Gneiss, chiefly quartz–feldspar–biotite; compositionally granitic
AI-generated identification · may be incorrectGray, coarse-grained, weakly foliated rock with visible quartz and feldspar and dark biotite/hornblende; quartz hardness 7, feldspar 6, biotite 2.5–3, typically nonmetallic luster and poor-to-absent cleavage as a whole.
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Physical properties
Gray, coarse-grained, weakly foliated rock with visible quartz and feldspar and dark biotite/hornblende; quartz hardness 7, feldspar 6, biotite 2.5–3, typically nonmetallic luster and poor-to-absent cleavage as a whole.
Formation & geological history
Formed when granitic or other felsic protolith was deeply buried and metamorphosed under high temperature and pressure, commonly during Precambrian to Paleozoic mountain building; surface weathering has rounded the specimen.
Uses & applications
Used as crushed aggregate, riprap, building stone, and occasionally dimension stone; attractive foliated pieces are collected but are generally low-value.
Geological facts
The image shows a rounded cobble amid mixed sediment, so exact identification is limited; it may also be strongly weathered granite rather than true gneiss.
Field identification & locations
Look for alternating light quartz–feldspar and dark mineral bands or streaks, coarse interlocking grains, and a very hard texture that scratches glass. Common in continental shields, glacial deposits, river gravels, and mountain belts.
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