metamorphic
Tumbled gneiss (likely biotite gneiss)
Gneiss; quartz–feldspar–biotite assemblage
AI-generated identification · may be incorrectTypically Mohs 6–7, with alternating light quartz/feldspar and dark biotite or amphibole bands, gray-brown color, and a dull to satin luster. The elongated rounded shape indicates natural abrasion or deliberate tumbling; foliation is visible as streaky banding.
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Physical properties
Typically Mohs 6–7, with alternating light quartz/feldspar and dark biotite or amphibole bands, gray-brown color, and a dull to satin luster. The elongated rounded shape indicates natural abrasion or deliberate tumbling; foliation is visible as streaky banding.
Formation & geological history
Formed by high-grade regional metamorphism of granite or sedimentary protoliths, commonly during Precambrian to younger mountain-building events. The specimen was likely rounded in a river or shaped and polished by tumbling.
Uses & applications
Used as aggregate, dimension stone, landscaping rock, and decorative polished stone; attractive pieces are sold as curios or massage stones.
Geological facts
Gneissic banding records segregation and recrystallization under intense heat and pressure, often deep within continental crust.
Field identification & locations
Identify it by distinct compositional banding, hard quartz-rich surfaces, and mica flakes; it commonly occurs in continental shields and eroded mountain belts. Exact identification would require a fresh surface or petrographic testing.
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