metamorphic
Banded gneiss pebble
Gneiss, chiefly quartz–feldspar with dark biotite/hornblende bands
AI-generated identification · may be incorrectHardness about 6–7 Mohs; gray, rounded, fine-to-medium grained, with dark foliation bands and a dull to slightly silky luster. Its mineral grains are interlocked rather than arranged as distinct crystals, and cleavage is weak or absent.
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Physical properties
Hardness about 6–7 Mohs; gray, rounded, fine-to-medium grained, with dark foliation bands and a dull to slightly silky luster. Its mineral grains are interlocked rather than arranged as distinct crystals, and cleavage is weak or absent.
Formation & geological history
Formed when preexisting granite or sedimentary rock underwent intense heat and pressure, producing compositional banding; the specimen is likely a weathered glacial or stream pebble. Gneiss commonly records Precambrian to younger regional metamorphism.
Uses & applications
Used as crushed aggregate, building stone, paving material, and decorative slabs; attractive banded pieces are collected but are usually low-value.
Geological facts
The dark streaks represent concentrations of mafic minerals such as biotite or amphibole, while lighter areas are quartz and feldspar. Gneissic banding distinguishes it from more uniform granite.
Field identification & locations
Common in metamorphic terrains and glacial deposits worldwide, especially Scandinavia, Canada, the northeastern United States, India, and Brazil. Identify it by alternating light and dark mineral bands and its hard, nonporous texture; a close image cannot confirm exact composition.
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