metamorphic

Gneiss

Biotite–quartz–feldspar gneiss

AI-generated identification · may be incorrect

A coarse-grained, foliated metamorphic rock with alternating pale quartz/feldspar and dark biotite-rich bands. Quartz and feldspar have Mohs hardness 6–7, vitreous to pearly luster, poor cleavage overall, and typical specific gravity around 2.6–2.8.

Identified September 14, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when granite or other felsic rock underwent high-grade regional metamorphism, commonly at amphibolite- to granulite-facies conditions during Precambrian orogenic events. Its banding reflects mineral segregation under intense heat and pressure.

Uses & applications

Used as crushed aggregate, dimension stone, paving, landscaping rock, and decorative slabs; attractive pieces have modest ornamental or educational value.

Geological facts

The specimen visibly shows gneissic banding and coarse interlocking crystals, though the dark material may include biotite, amphibole, or weathered mafic minerals. Exact identification requires a fresh surface or thin-section examination.

Field identification & locations

Identify it by its alternating light and dark mineral bands, coarse crystals, and strong foliation; unlike layered sedimentary rock, the bands are recrystallized and interlocking. Common worldwide in ancient continental shields and mountain belts.