metamorphic
Banded gneiss cobble
Gneiss, commonly quartz–feldspar–mica gneiss
AI-generated identification · may be incorrectTypically hard (Mohs 6–7), coarse-grained, with gray quartz/feldspar-rich bands and rusty orange-brown iron-oxide staining or biotite layers. Foliated, crystalline texture; cleavage is weak overall, and fresh surfaces have a vitreous to dull luster.
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Physical properties
Typically hard (Mohs 6–7), coarse-grained, with gray quartz/feldspar-rich bands and rusty orange-brown iron-oxide staining or biotite layers. Foliated, crystalline texture; cleavage is weak overall, and fresh surfaces have a vitreous to dull luster.
Formation & geological history
Formed when granite or other crustal rock underwent high-grade metamorphism, causing mineral segregation into banding; commonly Precambrian to Paleozoic in age. This rounded specimen was likely transported and smoothed by glacial or stream action.
Uses & applications
Used as crushed stone, road aggregate, dimension stone, landscaping rock, and occasionally ornamental material; generally limited jewelry value.
Geological facts
The visible alternating light and dark bands reflect compositional layering produced during metamorphism rather than ordinary sedimentary bedding. Orange zones are likely weathered iron-bearing minerals.
Field identification & locations
Identify it by strong light–dark mineral banding, interlocking grains, and abundant quartz/feldspar; unlike granite, it shows planar foliation. Common in ancient continental shields and glacial deposits worldwide.
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