metamorphic
Gneiss (likely granitic gneiss)
Foliated quartz–feldspar gneiss, commonly derived from granite
AI-generated identification · may be incorrectTypically quartz, feldspar, biotite or hornblende; coarse-grained with irregular light–dark banding, pink feldspar, gray quartz, and dark mafic minerals. Hardness about 6–7 Mohs, vitreous to dull luster, massive to weakly foliated, with no prominent cleavage.
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Physical properties
Typically quartz, feldspar, biotite or hornblende; coarse-grained with irregular light–dark banding, pink feldspar, gray quartz, and dark mafic minerals. Hardness about 6–7 Mohs, vitreous to dull luster, massive to weakly foliated, with no prominent cleavage.
Formation & geological history
Formed when granite or another felsic rock underwent intense, high-grade regional metamorphism, recrystallizing and segregating minerals into bands. Such rocks commonly record Precambrian to Paleozoic crustal events, though age cannot be determined from the photo.
Uses & applications
Used as dimension stone, paving, landscaping rock, retaining walls, and aggregate; attractive polished pieces may be decorative but are rarely gemstones.
Geological facts
The broad compositional banding distinguishes gneiss from relatively uniform granite, although weathering and glacial abrasion can obscure it.
Field identification & locations
Look for alternating quartz/feldspar-rich and dark biotite- or amphibole-rich bands, coarse crystals, and a hard, durable texture. Common in ancient continental shields and metamorphic mountain belts worldwide.
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