metamorphic

Banded gneiss

Gneiss, commonly quartz–feldspar–biotite gneiss

AI-generated identification · may be incorrect

Typically medium- to coarse-grained, with alternating light quartz/feldspar bands and dark biotite- or amphibole-rich bands; Mohs hardness about 6–7, vitreous to pearly luster, weak cleavage inherited from mica, and specific gravity roughly 2.6–2.8.

Identified September 19, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when granite or other rocks undergo intense high-temperature, high-pressure metamorphism, causing mineral segregation into foliation and compositional bands. Gneisses range from Precambrian to younger ages; the pictured block appears naturally weathered and partly cut or shaped.

Uses & applications

Used as building stone, landscape boulders, paving, monument stone, and crushed aggregate; attractive patterned pieces may be sold as decorative architectural stone, though this specimen is unlikely to be jewelry grade.

Geological facts

The rusty brown zones are likely iron-oxide weathering along fractures, while the bright white areas are probably quartz and feldspar. Gneiss commonly preserves evidence of deep crustal deformation and can contain folded or contorted bands.

Field identification & locations

Identify it by coarse interlocking grains and distinct light-dark mineral banding, unlike uniformly speckled granite or thinly layered schist. Common occurrences include ancient continental shields worldwide, especially Scandinavia, Canada, the United States, India, Brazil, and Australia.