metamorphic
Gneiss
Gneiss (Metamorphic rock)
Hardness: 6-7 Mohs; Color: Salt and pepper (gray, white, black); Luster: Dull to vitreous; Structure: Foliated with distinct bands (gneissose banding); Cleavage: Poor, typically breaks along plane of foliation; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 Mohs
- Color
- Salt and pepper (gray, white, black)
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Salt and pepper (gray, white, black); Luster: Dull to vitreous; Structure: Foliated with distinct bands (gneissose banding); Cleavage: Poor, typically breaks along plane of foliation; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of igneous (granite) or sedimentary (shale/sandstone) protoliths under extreme heat and pressure, often during mountain-building events.
Uses & applications
Used primarily in construction for crushed stone, road building, and architectural facing or countertops (sold as 'granite' in the trade).
Geological facts
The Acasta Gneiss in Canada is considered one of the oldest known crustal rock formations on Earth, dating back approximately 4.03 billion years.
Field identification & locations
Identified by 'ribbon' or 'banded' appearance where light minerals (quartz/feldspar) and dark minerals (biotite/amphibole) separate. Commonly found in Precambrian shields and the cores of mountain ranges.
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