metamorphic

Weathered granitic gneiss

Foliated quartz–feldspar–mica gneiss, likely with biotite and amphibole

AI-generated identification · may be incorrect

Typically Mohs 6–7; coarse-grained, gray to brown, with white feldspar/quartz and dark biotite or amphibole. It has a granular crystalline texture and weak compositional banding rather than obvious cleavage; rusty areas are iron-oxide staining.

Identified September 14, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when granite or another felsic protolith underwent high-grade regional metamorphism, commonly during Precambrian or younger mountain-building events; surface weathering rounded the specimen and oxidized iron-bearing minerals.

Uses & applications

Used as crushed aggregate, dimension stone, landscaping rock, and occasionally as decorative building stone; this piece appears more suitable for collecting than jewelry.

Geological facts

The interlocking grains and light–dark mineral segregation distinguish gneiss from most sedimentary rocks. Brown-orange patches commonly represent limonite or hematite produced by weathering of biotite or sulfides.

Field identification & locations

Common in continental shield regions and ancient mountain belts worldwide, including Scandinavia, Canada, the northeastern United States, India, and Australia. Identify it by its coarse quartz–feldspar matrix, dark mica/amphibole, and streaky or banded appearance; a geologic test is needed to separate it confidently from granite.