metamorphic
Banded gneiss pebble
Gneiss, composed chiefly of quartz, feldspar, and mica/amphibole
AI-generated identification · may be incorrectHardness about 6–7 Mohs; gray, white, and dark mineral bands; dull to slightly glassy luster; granular crystalline texture with weak cleavage inherited from mica. The rounded shape indicates natural water abrasion or tumbling.
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Physical properties
Hardness about 6–7 Mohs; gray, white, and dark mineral bands; dull to slightly glassy luster; granular crystalline texture with weak cleavage inherited from mica. The rounded shape indicates natural water abrasion or tumbling.
Formation & geological history
Formed when preexisting igneous or sedimentary rock underwent intense heat and pressure, producing compositional banding and recrystallization, commonly during Precambrian to Paleozoic mountain-building. The specimen was later weathered and transported as a pebble.
Uses & applications
Used as crushed aggregate, dimension stone, paving, and landscaping material; attractive polished pieces are sold as inexpensive geological specimens.
Geological facts
Gneissic banding reflects segregation of light quartz-feldspar minerals from darker biotite, hornblende, or other mafic minerals. It is distinct from slate or schist because its bands are typically thicker and more compositionally defined.
Field identification & locations
Identify it by alternating light and dark crystalline bands, interlocking grains, and lack of sedimentary layering or glassy fracture. Common in ancient continental shields and glacial or river gravels; a hand lens can reveal quartz, feldspar, and dark mica.
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