metamorphic
Biotite-rich gneiss
Gneiss, likely quartz–feldspar–biotite gneiss
AI-generated identification · may be incorrectCoarse-grained, strongly foliated rock with dark biotite/hornblende, pale quartz and feldspar, and possible pegmatitic patches; hardness about 6–7, vitreous to pearly luster, poor cleavage overall but mica has perfect cleavage. The specimen appears weathered and partly fractured.
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Physical properties
Coarse-grained, strongly foliated rock with dark biotite/hornblende, pale quartz and feldspar, and possible pegmatitic patches; hardness about 6–7, vitreous to pearly luster, poor cleavage overall but mica has perfect cleavage. The specimen appears weathered and partly fractured.
Formation & geological history
Formed by high-grade metamorphism of granite or other felsic protoliths during deep burial, commonly during Precambrian–Paleozoic mountain building; exact age cannot be determined from the image.
Uses & applications
Used as dimension stone, decorative aggregate, landscaping rock, and occasionally countertops; attractive polished pieces have modest collecting value.
Geological facts
The alternating light and dark mineral bands reflect metamorphic segregation and deformation, not sedimentary layering. Gneiss commonly contains relict minerals from the original granite.
Field identification & locations
Identify it by coarse crystals and planar light–dark banding, with mica flakes that split into elastic sheets; common in ancient continental shields and metamorphic mountain belts worldwide. A hand lens and fresh break would improve identification.
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