metamorphic

Biotite-rich gneiss

Foliated quartz–feldspar–biotite gneiss

AI-generated identification · may be incorrect

Coarse-grained, dark gray rock with abundant reflective black-brown biotite mica and lighter quartz/feldspar bands or lenses; mica gives a platy sheen and foliation. Hardness is roughly 6–7 overall, with mica cleavage and moderate-to-high density.

Identified September 2, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when granite or another felsic protolith underwent high-grade regional metamorphism, recrystallizing and segregating minerals into foliation. Such rocks commonly formed during Precambrian to Paleozoic mountain-building events, though exact age requires laboratory analysis.

Uses & applications

Used locally as dimension stone, architectural facing, landscaping material, and aggregate; attractive polished pieces may be sold as decorative stone. It is generally less sought after for jewelry than crystalline minerals.

Geological facts

The alternating light and dark mineral layers record directed pressure and elevated temperature deep within the crust. Biotite-rich surfaces can resemble schist, but gneiss typically shows compositional banding and larger, more interlocking grains.

Field identification & locations

Common in ancient continental shields and metamorphic mountain belts, including Canada, Scandinavia, India, and the Appalachian region. Identify it by visible foliation, abundant mica sparkle, quartz/feldspar grains, and lack of vesicles; a streak test is usually white to gray.