metamorphic
Banded metamorphic river cobble, likely gneiss or metasedimentary rock
Quartz–feldspar–mica gneiss or quartzite with cross-cutting quartz veins
AI-generated identification · may be incorrectHardness about 6–7 Mohs; gray-brown, fine- to medium-grained, with dark compositional banding and pale quartz veins. Nonmetallic, dull to slightly vitreous luster; foliation/bedding is visible, but crystal cleavage is not evident at specimen scale.
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Physical properties
Hardness about 6–7 Mohs; gray-brown, fine- to medium-grained, with dark compositional banding and pale quartz veins. Nonmetallic, dull to slightly vitreous luster; foliation/bedding is visible, but crystal cleavage is not evident at specimen scale.
Formation & geological history
Formed when preexisting sedimentary or igneous rock was buried and subjected to elevated heat and pressure, then later fractured and veined by silica-rich fluids. Its exact geological age cannot be determined from the photograph; the rounded shape reflects river abrasion.
Uses & applications
Used mainly as landscaping stone, riprap, paving, and decorative aggregate; attractive polished pieces may be sold as cabinet specimens. Ordinary cobbles have minimal commercial value.
Geological facts
The pale lines are probably quartz veins cutting across darker mineral bands, indicating multiple geological events. The smooth oval form is an erosional transport feature, not its original rock shape.
Field identification & locations
Identify it by strong planar banding, contrasting light and dark minerals, and hard quartz-rich surfaces; a streak test, acid test, and thin section would distinguish gneiss from quartzite or banded sandstone. Common in gravel bars downstream from metamorphic terrains.
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