metamorphic
Granitic gneiss (weathered, rounded cobble)
Foliated granitoid; primarily quartz, feldspar, and mica (composition commonly approximates granite)
AI-generated identification · may be incorrectMedium- to coarse-grained, pink, white, gray, and black mineral patches with weak compositional banding; vitreous quartz, feldspar cleavage, mica flakes, and Mohs hardness about 6–7. Typically nonporous, dense, and lacking obvious crystal faces due to weathering and rounding.
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Physical properties
Medium- to coarse-grained, pink, white, gray, and black mineral patches with weak compositional banding; vitreous quartz, feldspar cleavage, mica flakes, and Mohs hardness about 6–7. Typically nonporous, dense, and lacking obvious crystal faces due to weathering and rounding.
Formation & geological history
Formed when an original granite or similar felsic intrusive rock underwent high-grade metamorphism, recrystallization, and deformation, commonly during Precambrian mountain-building; the specimen was later rounded by stream or glacial transport.
Uses & applications
Used as crushed stone, aggregate, dimension stone, landscaping material, and occasionally decorative facing; attractive pieces may be collected but are generally not gem-quality.
Geological facts
The dark and light streaks reflect segregation and alignment of mafic minerals and felsic minerals during metamorphism. Its rounded shape indicates substantial transport or abrasion after uplift and erosion.
Field identification & locations
Identify it by interlocking quartz-feldspar grains, pink potassium feldspar, dark biotite or amphibole, and subtle foliation; it is common in ancient continental shields and metamorphic mountain belts.
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