metamorphic
Banded Gneiss
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Reddish-pink with dark grey to black banding; Luster: Dull to pearly; Crystal structure: Coarse-grained, phaneritic; Cleavage: Poor, but splits along foliation layers; Specific Gravity: 2.6 - 2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Reddish-pink with dark grey to black banding
- Luster
- Dull to pearly
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Reddish-pink with dark grey to black banding; Luster: Dull to pearly; Crystal structure: Coarse-grained, phaneritic; Cleavage: Poor, but splits along foliation layers; Specific Gravity: 2.6 - 2.9.
Formation & geological history
Formed through high-pressure, high-temperature regional metamorphism of igneous (granite) or sedimentary protoliths. The distinct banding (gneissose banding) is caused by the segregation of mafic and felsic minerals during intense tectonic activity, often dating back to the Precambrian or Paleozoic eras.
Uses & applications
Used in construction as crushed stone or dimension stone for countertops and building facades. Tumbled or polished specimens are popular for amateur geology collections and decorative landscaping.
Geological facts
Gneiss is one of the oldest rock types found on Earth. Some of the oldest known crustal rocks, the Acasta Gneiss in Canada, are approximately 4.03 billion years old.
Field identification & locations
Identify in the field by looking for 'gneissose banding' where dark and light minerals form distinct parallel layers. It is ubiquitous in mountainous regions and glacial drift. Common in Canada, Scandinavia, and the Appalachian mountains.
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