metamorphic
Gneiss (Green Gneiss)
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 on Mohs scale. Color: Greenish-grey with white/grey bands. Luster: Dull to pearly. Crystal structure: Foliated/Gneissose texture with coarse grains. Cleavage: Poor, but splits along bands. Specific gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Greenish-grey with white/grey bands
- Luster
- Dull to pearly
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Greenish-grey with white/grey bands. Luster: Dull to pearly. Crystal structure: Foliated/Gneissose texture with coarse grains. Cleavage: Poor, but splits along bands. Specific gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary protoliths (likely granitic or volcanic) under intense heat and pressure. Often Pre-Cambrian to Paleozoic in age.
Uses & applications
Used primarily as a crushed stone for road construction, as dimension stone for building facades, floor tiles, and as decorative landscaping rock.
Geological facts
The Acasta Gneiss in Canada is one of the oldest known intact crustal fragments on Earth, dated to approximately 4 billion years ago. The distinct banding is called 'gneissose banding'.
Field identification & locations
Identify by its characteristic alternating dark and light mineral bands (foliation). Found globally in the cores of mountain ranges and ancient continental shields. Look for visible quartz, feldspar, and darker mica or amphibole layers.
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