metamorphic
Banded gneiss
Gneiss, commonly quartz–feldspar–biotite gneiss
AI-generated identification · may be incorrectTypically medium- to coarse-grained, with alternating light quartz/feldspar bands and dark biotite- or amphibole-rich layers; hardness about 6–7 Mohs, vitreous to dull luster, and weak to absent cleavage at specimen scale. The rounded shape appears naturally weathered or tumbled.
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Physical properties
Typically medium- to coarse-grained, with alternating light quartz/feldspar bands and dark biotite- or amphibole-rich layers; hardness about 6–7 Mohs, vitreous to dull luster, and weak to absent cleavage at specimen scale. The rounded shape appears naturally weathered or tumbled.
Formation & geological history
Formed when granite or other continental crust underwent high-grade regional metamorphism, producing compositional banding and recrystallization. Gneiss commonly records Precambrian to younger mountain-building events, though this specimen’s exact age cannot be determined visually.
Uses & applications
Used as dimension stone, tiles, countertops, aggregate, and decorative landscaping rock; attractive banded pieces are collected but are generally not gemstone material.
Geological facts
The light bands are chiefly quartz and feldspar, while dark bands commonly contain biotite, amphibole, or garnet. Strong mineral banding distinguishes gneiss from more uniformly textured granite.
Field identification & locations
Identify it by coarse crystals and distinct, often folded light–dark banding; it is common in ancient continental shields and metamorphic mountain belts worldwide. A hand lens and hardness/streak tests help confirm quartz and feldspar.
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