metamorphic
Gneiss pebble
Banded gneiss, commonly quartz–feldspar–biotite gneiss
AI-generated identification · may be incorrectTypically coarse-grained with alternating light quartz/feldspar and dark mica or amphibole bands; hardness about 6–7, vitreous to pearly luster, and no consistent cleavage as a whole. The rounded surface indicates stream or beach transport and abrasion.
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Physical properties
Typically coarse-grained with alternating light quartz/feldspar and dark mica or amphibole bands; hardness about 6–7, vitreous to pearly luster, and no consistent cleavage as a whole. The rounded surface indicates stream or beach transport and abrasion.
Formation & geological history
Formed when granite or other rocks underwent intense metamorphism, causing minerals to recrystallize and segregate into compositional bands; many gneisses are Precambrian, commonly over 1 billion years old. The specimen was later weathered and rounded in a sedimentary environment.
Uses & applications
Used as crushed stone, aggregate, dimension stone, landscaping rock, and occasionally decorative countertop or facing material; this small pebble has mainly collecting value.
Geological facts
Its banding records high-temperature and pressure conditions deep in the continental crust. The pale layers are likely quartz and feldspar, while dark layers may contain biotite, hornblende, or other ferromagnesian minerals.
Field identification & locations
Identify it by coarse interlocking grains and irregular light–dark banding, unlike the thin foliation of schist or uniform granite. Common in glacial deposits, mountain streams, and exposed Precambrian terrains worldwide.
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