metamorphic
Banded Gneiss
Gneiss
Hardness: 6-7 Mohs. Color: Alternating light (quartz/feldspar) and dark (biotite/amphibole) mineral bands. Luster: Dull to vitreous. Texture: Medium to coarse-grained foliated structure with distinct gneissic banding. Features a small micro-fault displacement visible across the bands.
- Hardness
- 6-7 Mohs
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 Mohs. Color: Alternating light (quartz/feldspar) and dark (biotite/amphibole) mineral bands. Luster: Dull to vitreous. Texture: Medium to coarse-grained foliated structure with distinct gneissic banding. Features a small micro-fault displacement visible across the bands.
Formation & geological history
Formed under high-grade regional metamorphism involving intense heat and pressure that recrystallized pre-existing igneous or sedimentary rocks, causing mineral segregation into distinct compositional bands.
Uses & applications
Used as crushed stone in construction, road aggregates, decorative landscaping stone, dimension stone for countertops and building facades, and as educational geological specimens.
Geological facts
This specimen clearly displays a micro-fault, where structural stresses offset the light and dark bands after or during metamorphic recrystallization. Gneiss includes some of the oldest rocks on Earth, such as the Acasta Gneiss in Canada, dated to around 4.03 billion years old.
Field identification & locations
Identified in the field by its pronounced alternating light and dark parallel banding (gneissic foliation) and crystalline texture. Commonly found in exposed Precambrian shields, mountain cores, and glacial river cobbles globally.
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