metamorphic
Banded Gneiss
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Alternating light (quartz/feldspar) and dark (biotite/hornblende) bands; Luster: Dull to vitreous; Structure: Foliated with distinct mineral segregation; Cleavage: Poor (tends to break along bands).
- 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 (quartz/feldspar) and dark (biotite/hornblende) bands; Luster: Dull to vitreous; Structure: Foliated with distinct mineral segregation; Cleavage: Poor (tends to break along bands).
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme pressure and temperatures (above 600°C). This specific specimen has been water-worn into a rounded cobble, likely in a river or glacial environment.
Uses & applications
Commonly used as crushed stone for road construction and concrete aggregate. High-quality slabs are used for countertops, flooring, and building facades due to its durability and aesthetic banding.
Geological facts
Gneiss contains some of the oldest known rock crust on Earth, such as the Acasta Gneiss in Canada, which is approximately 4.03 billion years old.
Field identification & locations
Identify in the field by looking for 'gneissic banding'—distinctive layers of different minerals that do not easily split like slate or schist. It is commonly found in the roots of mountain ranges and ancient continental shields like the Canadian Shield or the Appalachian Mountains.
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