metamorphic
Banded gneiss (likely amphibolite-rich gneiss)
Banded gneiss; composition commonly quartz, feldspar, mica and/or amphibole
AI-generated identification · may be incorrectDark gray to black, with pale mineral bands and a foliated, layered texture; typically Mohs 6–7, vitreous to dull luster, granular crystals, and poor or absent cleavage as a whole rock. The specimen appears cut or naturally slab-like, so exact identification is limited by the image.
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Physical properties
Dark gray to black, with pale mineral bands and a foliated, layered texture; typically Mohs 6–7, vitreous to dull luster, granular crystals, and poor or absent cleavage as a whole rock. The specimen appears cut or naturally slab-like, so exact identification is limited by the image.
Formation & geological history
Formed when preexisting igneous or sedimentary rock underwent high-grade metamorphism, causing minerals to recrystallize and segregate into bands. Gneisses commonly formed during Precambrian to younger mountain-building events.
Uses & applications
Used as dimension stone, countertops, paving and landscaping rock; attractive banded pieces are collected as geological specimens. It is generally not a jewelry gemstone.
Geological facts
The alternating light and dark bands are called gneissic banding and record intense heat, pressure and deformation. Dark bands may be amphibole- or biotite-rich, while light bands are commonly quartz- and feldspar-rich.
Field identification & locations
Identify it by coarse interlocking grains and persistent compositional banding, unlike flat slate cleavage or uniform basalt. Common worldwide in ancient continental shields and metamorphic mountain belts.
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