metamorphic

Banded gneiss

Gneiss, commonly quartz–feldspar–mica gneiss

AI-generated identification · may be incorrect

Typically medium- to coarse-grained, with alternating pale quartz/feldspar and dark biotite or amphibole bands; hardness about 6–7 Mohs, vitreous to pearly luster, weak foliation, and no consistent cleavage as a whole rock.

Identified September 7, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed by high-grade regional metamorphism of granite or sedimentary protoliths, commonly during Precambrian to Paleozoic mountain-building; the visible specimen is a weathered, rounded glacial or stream cobble.

Uses & applications

Used as dimension stone, paving, aggregate, landscaping rock, and occasionally as decorative facing; attractive polished pieces are collected but are generally low value.

Geological facts

The pronounced compositional banding distinguishes gneiss from more evenly foliated schist. Its light bands are usually quartz and feldspar, while dark streaks commonly contain biotite or hornblende.

Field identification & locations

Common in continental shields and exposed mountain belts, including New England, the Canadian Shield, Scandinavia, and the Alps. Field identification relies on coarse grains and irregular light–dark banding; acid reaction should be absent unless carbonate veins occur.