metamorphic
Biotite gneiss (probable)
Foliated gneiss composed mainly of quartz, feldspar, and biotite mica
AI-generated identification · may be incorrectTypically gray, coarse-grained, and banded or weakly foliated, with a sparkly mica luster; hardness about 6–7, no consistent cleavage as a whole, and specific gravity roughly 2.6–2.8. The pictured stone appears weathered and somewhat schistose, so exact identification is uncertain from the image alone.
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Physical properties
Typically gray, coarse-grained, and banded or weakly foliated, with a sparkly mica luster; hardness about 6–7, no consistent cleavage as a whole, and specific gravity roughly 2.6–2.8. The pictured stone appears weathered and somewhat schistose, so exact identification is uncertain from the image alone.
Formation & geological history
Formed when granite or other sedimentary/igneous protoliths underwent intense heat and pressure, commonly during Precambrian to Paleozoic mountain building. It is widespread in exposed continental basement and glacial deposits.
Uses & applications
Used as crushed aggregate, road stone, landscaping rock, and occasionally dimension stone; attractive polished pieces may be decorative but are rarely used as gemstones.
Geological facts
The alternating light and dark mineral texture reflects metamorphic segregation, with quartz-feldspar-rich layers contrasting with biotite-rich layers. Gneiss commonly records deep crustal conditions and may preserve relics of older rocks.
Field identification & locations
Look for coarse interlocking grains and compositional banding; quartz scratches glass, while dark mica flakes may separate into thin sheets. Common in mountain belts and glacial tills, including parts of New England, Canada, Scandinavia, and the upper Midwest.
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