metamorphic
Banded Gneiss
Gneiss (SiO2-rich metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Light gray to tan with prominent dark brownish-black or dark gray bands; Luster: Dull to vitreous; Texture: Medium to coarse-grained with distinct foliation (gneissic banding); Cleavage: Poor, fractures unevenly.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Light gray to tan with prominent dark brownish-black or dark gray bands; Luster: Dull to vitreous; Texture: Medium to coarse-grained with distinct foliation (gneissic banding); Cleavage: Poor, fractures unevenly.
Formation & geological history
Formed via high-grade regional metamorphism of pre-existing sedimentary rocks (like shale or sandstone) or igneous rocks (like granite) under extreme temperature and pressure conditions deep within the Earth's crust.
Uses & applications
Widely used as an ornamental stone, landscape boulder, building cladding, crushed stone for construction, and as decorative river/garden rock.
Geological facts
The alternating light and dark bands are formed by mineral segregation during metamorphism—light bands are rich in quartz and feldspar, while dark bands consist mainly of biotite mica or amphibole. Gneiss includes some of the oldest rocks on Earth, such as the Acasta Gneiss in Canada (over 4 billion years old).
Field identification & locations
Identified in the field by its characteristic alternating light and dark mineral bands (gneissic layering) without a strong tendency to split along planes like schist. Commonly found in exposed ancient continental shields and mountain belts worldwide.
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