metamorphic
Banded gneiss (likely weathered paragneiss)
Gneiss; quartz–feldspar–mica ± amphibole assemblage
AI-generated identification · may be incorrectTypically hardness 6–7 Mohs, gray to blue-gray with pale quartz/feldspar bands and dark mica or amphibole; vitreous to dull luster, granular crystalline texture, and no prominent cleavage as a whole. The rounded shape and surface fractures indicate stream or shoreline abrasion.
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Physical properties
Typically hardness 6–7 Mohs, gray to blue-gray with pale quartz/feldspar bands and dark mica or amphibole; vitreous to dull luster, granular crystalline texture, and no prominent cleavage as a whole. The rounded shape and surface fractures indicate stream or shoreline abrasion.
Formation & geological history
Formed when preexisting igneous or sedimentary rock underwent high-grade regional metamorphism, commonly at about บ 600–800°C and elevated pressure during mountain building. Individual specimens may be Precambrian to Paleozoic or younger, depending on locality.
Uses & applications
Used as crushed aggregate, dimension stone, landscape rock, and occasionally decorative slabs; usually modest value as a hand specimen.
Geological facts
The light and dark layers represent mineral segregation and compositional banding, not sedimentary bedding. Pale veins likely consist of quartz or feldspar introduced or remobilized during deformation.
Field identification & locations
Identify it by alternating light and dark mineral bands, interlocking grains, and strong foliation; distinguish it from slate or phyllite by its coarse grains and lack of smooth splitting. Common in ancient continental shields and metamorphic mountain belts.
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