metamorphic
Banded Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Dark grey to black matrix with lighter grey or white bands; Luster: Dull to sub-vitreous; Crystal Structure: Coarse-grained with distinct foliation (gneissic banding); Cleavage: Poor, usually breaks along uneven surfaces.
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey to black matrix with lighter grey or white bands
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey to black matrix with lighter grey or white bands; Luster: Dull to sub-vitreous; Crystal Structure: Coarse-grained with distinct foliation (gneissic banding); Cleavage: Poor, usually breaks along uneven surfaces.
Formation & geological history
Formed through high-pressure, high-temperature metamorphism (regional metamorphism) of igneous or sedimentary protoliths, often during mountain-building events deep within the Earth's crust.
Uses & applications
Commonly used as a crushed stone for road construction, as building stone (facing, curbing), and occasionally as decorative landscaping stone or as a collector's specimen if exhibiting striking patterns.
Geological facts
Gneiss contains some of the oldest rocks on Earth, such as the Acasta Gneiss in Canada, dated to approximately 4 billion years old. The white bands are typically composed of light minerals like quartz or feldspar, while dark bands contain biotite or hornblende.
Field identification & locations
Identify in the field by looking for distinct, alternating bands of light and dark minerals that are often folded or wavy. Found globally in continental shield areas and mountain ranges like the Appalachians or Himalayas.
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