metamorphic
Banded gneiss cobble
Gneiss, commonly quartz–feldspar–biotite gneiss
AI-generated identification · may be incorrectTypically 6–7 Mohs; coarse-grained, nonfoliated-to-strongly foliated bands of light quartz/feldspar and dark biotite or hornblende, with vitreous to pearly luster and poor cleavage as a whole. The rounded shape indicates glacial or stream transport and weathering.
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Physical properties
Typically 6–7 Mohs; coarse-grained, nonfoliated-to-strongly foliated bands of light quartz/feldspar and dark biotite or hornblende, with vitreous to pearly luster and poor cleavage as a whole. The rounded shape indicates glacial or stream transport and weathering.
Formation & geological history
Forms when granite or other high-grade protoliths undergoes intense heat and pressure, causing mineral segregation into bands, commonly during Precambrian to Paleozoic mountain-building. This specimen was likely rounded from glacial abrasion or river transport.
Uses & applications
Used as decorative landscaping stone, aggregate, building veneer, and occasionally polished as an ornamental stone; generally not a jewelry gemstone.
Geological facts
Gneissic banding is metamorphic layering, not sedimentary bedding. Dark patches may be biotite-rich or amphibole-rich zones, while white areas are mostly quartz and feldspar.
Field identification & locations
Look for alternating coarse light and dark mineral bands, interlocking grains, and no obvious glassy or vesicular texture; a knife should not scratch it easily. Common in ancient continental shields and glacial deposits worldwide.
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