metamorphic
Banded Gneiss
Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 (Mohs scale); Color: Gray, white, and black; Luster: Dull to pearly; Structure: Foliated with distinct bands (gneissic banding); Cleavage: Poor; Specific gravity: 2.6-2.9.
- Hardness
- 6-7 (Mohs scale)
- Color
- Gray, white, and black
- Luster
- Dull to pearly
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Gray, white, and black; Luster: Dull to pearly; Structure: Foliated with distinct bands (gneissic banding); Cleavage: Poor; Specific gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks at extreme temperatures and pressures deep within the Earth's crust. Most specimens are Proterozoic or Archean in age.
Uses & applications
Commonly used as a crushed stone in construction, dimension stone for buildings and countertops, and as aesthetic landscaping or river stones.
Geological facts
Gneiss contains some of the oldest known rock units on Earth, such as the Acasta Gneiss in Canada, which is over 4 billion years old.
Field identification & locations
Identify in the field by its distinct alternating layers of light-colored (quartz/feldspar) and dark-colored (biotite/amphibole) minerals. Found in mountainous or shield regions.
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