metamorphic
Banded Gneiss
Gneiss (Metamorphic Rock)
Hardness: 6-7 (Mohs); Color: Alternating dark and light bands (mafic and felsic); Luster: Dull to pearly; Crystal structure: Granular/Coarse; Cleavage: Poor, but exhibits foliation.
- Hardness
- 6-7 (Mohs)
- Color
- Alternating dark and light bands (mafic and felsic)
- Luster
- Dull to pearly
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Physical properties
Hardness: 6-7 (Mohs); Color: Alternating dark and light bands (mafic and felsic); Luster: Dull to pearly; Crystal structure: Granular/Coarse; Cleavage: Poor, but exhibits foliation.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme heat and pressure. Often billions of years old (Precambrian).
Uses & applications
Used as building stone, decorative landscaping rock, crushed stone for road construction, and occasionally for high-end countertops.
Geological facts
The Acasta Gneiss in Canada is one of the oldest known crustal rocks on Earth, dated at approximately 4.03 billion years old.
Field identification & locations
Identify by distinct 'gneissic banding' where minerals are segregated into visible layers. Common in continental shields and mountain cores. Look for light bands of quartz/feldspar and dark bands of biotite/amphibole.
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