metamorphic
Felsic gneiss, likely granitic gneiss
Quartz–feldspar–mica gneiss; commonly quartz + plagioclase/K-feldspar + biotite or hornblende
AI-generated identification · may be incorrectHardness about 6–7 Mohs; coarse-grained, gray to black and cream, with sparkly mica and irregular light/dark compositional banding. Nonmetallic luster, interlocking crystalline texture, and generally poor cleavage as a rock.
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Physical properties
Hardness about 6–7 Mohs; coarse-grained, gray to black and cream, with sparkly mica and irregular light/dark compositional banding. Nonmetallic luster, interlocking crystalline texture, and generally poor cleavage as a rock.
Formation & geological history
Formed when granite or another felsic protolith underwent high-grade regional metamorphism, typically during Precambrian to Paleozoic mountain-building; exact age cannot be determined from the image.
Uses & applications
Used as crushed aggregate, dimension stone, paving, and landscaping rock; attractive pieces may be polished for decorative use, though this small specimen has little commercial value.
Geological facts
The alternating light and dark domains reflect mineral segregation during intense heat and pressure, not sedimentary layering.
Field identification & locations
Common in ancient continental shields and metamorphic mountain belts; identify by visible quartz/feldspar grains, mica sparkle, and compositional banding. Similar dark rocks may be amphibolite or diorite, requiring a fresh surface or laboratory testing for certainty.
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