metamorphic
Tumbled gneiss
Gneiss, chiefly quartz–feldspar with mica and dark ferromagnesian minerals
AI-generated identification · may be incorrectTypically 6–7 Mohs; gray to black with conspicuous light/dark banding, granular texture, and vitreous to dull luster. It has no consistent cleavage as a rock, though constituent feldspar and mica may cleave; density is about 2.6–2.8 g/cm³.
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Physical properties
Typically 6–7 Mohs; gray to black with conspicuous light/dark banding, granular texture, and vitreous to dull luster. It has no consistent cleavage as a rock, though constituent feldspar and mica may cleave; density is about 2.6–2.8 g/cm³.
Formation & geological history
Gneiss forms when granite or other high-grade protoliths undergo intense heat and pressure, producing mineral segregation into bands, commonly during Precambrian to Phanerozoic mountain-building. This rounded specimen appears water-worn or mechanically tumbled.
Uses & applications
Used as crushed aggregate, road stone, riprap, building stone, and decorative landscaping material; attractive polished pieces are sold as desk or collection stones.
Geological facts
Its alternating bands record metamorphic deformation and recrystallization rather than sedimentary layering. Individual mineral identification may require a hand lens or thin section.
Field identification & locations
Identify it by coarse interlocking grains and irregular light/dark compositional banding; quartz is commonly glassy, feldspar pale, and biotite or amphibole dark. Common in continental shields and ancient mountain belts worldwide.
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