metamorphic

Biotite-bearing gneiss

Mica gneiss, composed mainly of quartz, feldspar, and biotite mica

AI-generated identification · may be incorrect

Gray, coarse-grained, foliated rock with sparkly black-brown biotite and pale quartz/feldspar bands. Typical hardness is about 6–7; vitreous quartz and feldspar contrast with pearly mica cleavage, and specific gravity is roughly 2.6–2.8.

Identified September 7, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when preexisting igneous or sedimentary rock underwent intense heat and pressure, producing mineral segregation and foliation. Its exact geological age cannot be determined from the photograph; gneisses commonly range from Precambrian to younger ages.

Uses & applications

Used locally as dimension stone, decorative aggregate, landscaping rock, and building material; attractive pieces are collected, though it is not generally a jewelry stone.

Geological facts

Gneiss is identified by alternating light and dark mineral bands or streaks and represents high-grade metamorphism. Biotite flakes may reflect light strongly and split into thin elastic sheets.

Field identification & locations

Common in ancient continental shield regions and metamorphic mountain belts, including Canada, the northeastern United States, Scandinavia, and India. Look for foliation, abundant feldspar/quartz, and black mica rather than a uniform granite texture.