metamorphic

Biotite-bearing gneiss (probable)

Foliated metamorphic rock composed chiefly of quartz, feldspar, and dark biotite/amphibole

AI-generated identification · may be incorrect

Hardness about 6–7 Mohs; coarse-grained, mottled gray, cream, brown, and black; commonly displays weak compositional banding and a glassy-to-pearly luster. Individual minerals show no obvious crystal form in this weathered specimen; density is typically 2.6–2.8 g/cm³.

Identified September 1, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when preexisting granite or sedimentary rock underwent intense heat and pressure, recrystallizing and segregating minerals into bands, commonly during Precambrian to Paleozoic mountain-building. Exact identification is limited by the photograph.

Uses & applications

Used as crushed stone, aggregate, road base, dimension stone, and occasionally decorative facing; attractive polished pieces are collected but usually have modest value.

Geological facts

Gneiss is identified by mineral banding rather than the thin layering typical of schist. The dark reflective grains are likely biotite mica or amphibole, while pale grains are quartz and feldspar.

Field identification & locations

Look for alternating light and dark mineral bands, coarse interlocking grains, and substantial hardness; a streak, thin-section, or fresh fracture would improve identification. Common worldwide in ancient continental shields and mountain belts.