metamorphic
Gneiss (specifically a granitic gneiss cobble)
Gneiss (high-grade regional metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Banded salt-and-pepper appearance with pink/tan/white and black; Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding (gneissic banding); Mineralogy: Quartz, Feldspar, and Biotite/Amphibole.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded salt-and-pepper appearance with pink/tan/white and black; Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding (gneissic banding); Mineralogy: Quartz, Feldspar, and Biotite/Amphibole.
Formation & geological history
Formed through high-grade regional metamorphism of igneous (granite) or sedimentary protoliths. This process occurs under extreme pressure and temperature deep within the Earth's crust, often during mountain-building events (orogenies). This specimen appears water-worn, likely a glacial or river-transported cobble.
Uses & applications
Used primarily in construction as crushed stone for road bases, concrete aggregate, and railroad ballast. High-quality slabs are used for architectural facing, countertops (often sold as 'granite'), and paving.
Geological facts
Gneiss contains some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada, which is approximately 4.03 billion years old. The term 'gneiss' comes from a Middle High German verb meaning 'to spark' because the rock can glint in the light.
Field identification & locations
Identify in the field by looking for 'gneissic banding'—alternating layers of light-colored minerals (quartz/feldspar) and dark minerals (biotite/hornblende). Unlike schist, it does not split easily along planes. Commonly found in the crystalline cores of mountain ranges and glacial till.
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