metamorphic
Banded Gneiss
Gneiss (Foliated Metamorphic Rock)
Hardness: 6-7 on Mohs scale. Color: Alternating light-colored bands (quartz, feldspar) and dark-colored bands (biotite, hornblende, amphibole). Luster: Dull to vitreous. Texture: Medium- to coarse-grained, distinctly foliated with gneissic banding.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Alternating light-colored bands (quartz, feldspar) and dark-colored bands (biotite, hornblende, amphibole). Luster: Dull to vitreous. Texture: Medium- to coarse-grained, distinctly foliated with gneissic banding.
Formation & geological history
Formed by high-grade regional metamorphism under intense heat and directional pressure, usually deep within collisional mountain belts. Precursor (protolith) is often granite, diorite, or sedimentary rocks like shale.
Uses & applications
Widely used as crushed stone for road construction, railway ballast, aggregate, and as a decorative building stone, countertops, or landscaping cobbles.
Geological facts
The world's oldest known intact rock units, such as the Acasta Gneiss in northern Canada, are made of gneiss and date back to approximately 4.03 billion years ago.
Field identification & locations
Identified in the field by prominent, coarse-grained compositional banding where light mineral bands alternate with dark mineral bands. Commonly found in riverbeds, glacial tills, and mountain ranges composed of ancient crystalline basement rocks.
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