metamorphic

Biotite-rich gneiss or schist

Mica gneiss/schist; primarily quartz, feldspar, biotite, and possibly amphibole

AI-generated identification · may be incorrect

Dark gray to black, coarse-grained, strongly foliated, with a micaceous sheen and irregular fracture; typical hardness is about 2.5–7 depending on the mineral tested, and specific gravity is roughly 2.7–3.0. Individual mica flakes show perfect basal cleavage, while quartz and feldspar are harder and non-cleaving.

Identified September 16, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when preexisting igneous or sedimentary rock underwent high-pressure, high-temperature metamorphism, producing aligned mica layers and compositional banding; commonly Precambrian to Paleozoic in exposed continental mountain belts.

Uses & applications

Used locally as decorative stone, aggregate, dimension stone, and educational specimens; attractive polished pieces may be collected but are not usually gemstone grade.

Geological facts

The layered texture records directed tectonic pressure and can cause the rock to split along foliation. Abundant biotite or hornblende gives it a darker appearance than typical light-colored gneiss.

Field identification & locations

Identify it by visible foliation, sparkling dark mica, and alternating coarse mineral bands; distinguish it from slate by its coarser crystals and from basalt by its planar fabric. Common in ancient crystalline terrains and mountain ranges worldwide.