metamorphic
Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Banded appearance with white, gray, and reddish-pink layers; Luster: Dull to vitreous; Structure: Foliated (gneissic banding); Cleavage: Poorly defined; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Banded appearance with white, gray, and reddish-pink layers
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded appearance with white, gray, and reddish-pink layers; Luster: Dull to vitreous; Structure: Foliated (gneissic banding); Cleavage: Poorly defined; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary protoliths under extreme temperature and pressure. It often dates back to the Precambrian era.
Uses & applications
Used as a decorative building stone, in road construction as crushed stone, and occasionally for countertops or landscaping. Some attractive specimens are polished by rockhounds.
Geological facts
Gneiss is some of the oldest rock on Earth, with the Acasta Gneiss in Canada dated at over 4 billion years old. The banding occurs due to 'metamorphic differentiation' of minerals.
Field identification & locations
Identify by its 'gneissic banding' where dark and light minerals separate into layers. Commonly found in the cores of mountain ranges or glacial till. This specimen suggests erosion and tumbling in a river or beach environment.
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