metamorphic

Polished granitic gneiss pebble

Quartz–feldspar–biotite gneiss, possibly granitic gneiss

AI-generated identification · may be incorrect

Hardness about 6–7 Mohs; gray, white, brown, and black mottling with a dull to slightly vitreous luster. Coarse interlocking quartz and feldspar grains, dark biotite/hornblende, and weak compositional banding are visible; cleavage is absent overall, though feldspar and mica may cleave.

Identified September 3, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when granitic rock underwent high-grade regional metamorphism, causing recrystallization and mineral segregation; the exact age cannot be determined from the image and may range from Precambrian to younger. The rounded shape indicates stream or beach abrasion, likely as a naturally tumbled pebble.

Uses & applications

Used as crushed stone, aggregate, dimension stone, landscaping material, and decorative pebbles; attractive pieces may be sold as cabinet specimens or craft stones.

Geological facts

Gneiss commonly preserves evidence of intense heat and pressure deep in continental crust. Its light–dark mineral banding distinguishes it from many unfoliated granites.

Field identification & locations

Common in ancient continental shields and metamorphic mountain belts; identify by coarse grains, quartz-rich gray areas, feldspar, dark mica, and streaky banding. A hand lens and fresh fracture are more diagnostic than the weathered surface.