metamorphic
Dark gneiss or banded metamorphic rock
Banded quartz–feldspar–mica gneiss; exact identification requires testing
AI-generated identification · may be incorrectFine- to medium-grained, dark gray, with a pale linear mineral band and subdued vitreous-to-dull luster. Likely hardness about 6–7, with quartz/feldspar grains and weak foliation; cleavage is absent to poor and specific gravity roughly 2.6–2.8.
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Physical properties
Fine- to medium-grained, dark gray, with a pale linear mineral band and subdued vitreous-to-dull luster. Likely hardness about 6–7, with quartz/feldspar grains and weak foliation; cleavage is absent to poor and specific gravity roughly 2.6–2.8.
Formation & geological history
Formed when preexisting igneous or sedimentary rock underwent high-pressure, high-temperature metamorphism, causing minerals to segregate into bands. Its precise geological age cannot be determined from the photograph.
Uses & applications
Used locally as aggregate, paving, building stone, and decorative material; attractive pieces may be sold as polished or natural specimens. This small rounded fragment has little commercial value.
Geological facts
The pale stripe may be a quartz or feldspar vein rather than true gneissic banding. Weathering and abrasion have rounded the specimen, obscuring its original fabric.
Field identification & locations
Common in continental mountain belts and glaciated terrains. Look for alternating light/dark mineral bands, visible interlocking grains, and a streak that does not powder strongly; a glass-scratch test and acid test can help distinguish quartz-rich gneiss from basalt or slate.
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