metamorphic

Biotite–quartz–feldspar gneiss

Gneiss, composed chiefly of quartz (SiO₂), feldspar, and dark mica, likely biotite

AI-generated identification · may be incorrect

Typically 6–7 Mohs; gray to black with white crystalline bands and rusty brown weathering. Vitreous quartz/feldspar luster, platy mica sparkle, no strong cleavage as a whole, and specific gravity about 2.6–2.8; visible compositional banding is characteristic.

Identified August 11, 2026More metamorphic →

Explore Biotite–quartz–feldspar gneiss in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed by high-grade metamorphism of granite or other felsic protoliths, causing recrystallization and mineral segregation into light and dark layers. Such rocks commonly formed during Precambrian to Paleozoic mountain-building events, though exact age requires laboratory analysis.

Uses & applications

Used as building stone, architectural slabs, landscaping rock, and aggregate; attractive pieces may be collected but are rarely suitable as gemstones.

Geological facts

The specimen shows coarse interlocking grains and probable gneissic banding, with dark biotite-rich zones and pale quartz–feldspar veins. It may be confused with coarse granite or migmatite if banding is weak.

Field identification & locations

Identify it by alternating light and dark mineral-rich bands, abundant mica sparkle, and hard quartz grains; a glass scratch test and inspection of fresh surfaces help distinguish it from softer schist. Common in ancient continental shields and metamorphic mountain belts worldwide.