metamorphic

Banded gneiss cobble

Gneiss, typically quartz–feldspar–mica rock

AI-generated identification · may be incorrect

Hardness about 6–7 Mohs; light gray to cream with dark biotite/amphibole bands, granular texture, and dull to weakly glassy luster. It has no prominent cleavage as a whole, though mica layers may split; density is roughly 2.6–2.8 g/cm³.

Identified September 19, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when preexisting granite or other felsic rock underwent intense regional metamorphism, producing compositional banding and recrystallization. Such rocks commonly formed during Precambrian to Paleozoic mountain-building events, though the exact age cannot be determined from the image.

Uses & applications

Used as decorative landscape stone, building and facing stone, countertops, monuments, and aggregate; attractive polished pieces have modest lapidary and collector appeal.

Geological facts

The dark streaks are concentrations of ferromagnesian minerals, while the pale areas are mainly quartz and feldspar. The rounded shape indicates prolonged transport and abrasion in a river, beach, or glacial deposit.

Field identification & locations

Identify it by coarse interlocking grains and alternating light–dark mineral bands; unlike granite, the bands are generally aligned or streaked. Common in glaciated terrains and ancient mountain belts worldwide.