metamorphic
Black-and-white gneiss
Gneiss, chiefly quartz–feldspar with dark biotite/amphibole bands
AI-generated identification · may be incorrectMedium-grained, pale gray to cream rock with irregular dark mineral banding; vitreous to dull luster and massive texture. Hardness is approximately 6–7 Mohs; feldspar cleavage may be visible, but the rounded specimen has weathered edges.
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Physical properties
Medium-grained, pale gray to cream rock with irregular dark mineral banding; vitreous to dull luster and massive texture. Hardness is approximately 6–7 Mohs; feldspar cleavage may be visible, but the rounded specimen has weathered edges.
Formation & geological history
Formed by high-grade metamorphism of granite or other felsic rocks, causing mineral segregation into light and dark bands. Such rocks commonly formed deep in Precambrian to younger continental crust, often hundreds of millions to billions of years old.
Uses & applications
Used as dimension stone, paving, landscaping aggregate, and decorative rock; attractive polished pieces are collected but are generally not gemstones.
Geological facts
The dark portions are likely biotite mica and/or hornblende, while the light matrix is quartz and feldspar. Strong compositional banding distinguishes gneiss from more uniformly speckled granite.
Field identification & locations
Identify it by alternating light quartz-feldspar and dark mica/amphibole bands, unlike concrete or glass; common in continental mountain belts and glaciated regions. A definitive identification may require a fresh surface or thin-section study.
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