metamorphic
Tumbled gneiss or granitic gneiss pebble
Quartz–feldspar–mica gneiss; composition typically quartz, plagioclase/alkali feldspar, and biotite or hornblende
AI-generated identification · may be incorrectHardness about 6–7 Mohs; rounded, weathered surface with tan feldspar-rich areas and dark mafic mineral bands. Generally vitreous to dull luster, granular crystal structure, and no obvious cleavage in the whole specimen.
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Physical properties
Hardness about 6–7 Mohs; rounded, weathered surface with tan feldspar-rich areas and dark mafic mineral bands. Generally vitreous to dull luster, granular crystal structure, and no obvious cleavage in the whole specimen.
Formation & geological history
Likely formed when granitic rock underwent high-grade regional metamorphism, producing compositional banding; the exact geological age cannot be determined from the photograph. Its rounded shape indicates prolonged stream, beach, or glacial abrasion.
Uses & applications
Used mainly as a landscaping, garden, or decorative stone; attractive polished pieces may be used as paperweights or lapidary specimens.
Geological facts
The dark streaks probably represent biotite, hornblende, or other mafic minerals, while the lighter zones are feldspar and quartz. The image cannot distinguish confidently between gneiss and a strongly banded granite.
Field identification & locations
Look for alternating light and dark mineral bands, interlocking grains, and a lack of sedimentary layering; a fresh surface and hand lens help confirm identification. Common as glacial or stream cobbles in many continental regions.
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