metamorphic
Gneiss
Gneiss (High-Grade Metamorphic Rock)
Hardness: 6-7 Mohs; Color: Banded grey, white, and tan; Luster: Pearly to dull; Texture: Foliated (gneissic banding); Composition: Quartz, feldspar, and darker minerals like biotite or hornblende.
- Hardness
- 6-7 Mohs
- Color
- Banded grey, white, and tan
- Luster
- Pearly to dull
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Physical properties
Hardness: 6-7 Mohs; Color: Banded grey, white, and tan; Luster: Pearly to dull; Texture: Foliated (gneissic banding); Composition: Quartz, feldspar, and darker minerals like biotite or hornblende.
Formation & geological history
Formed through high-pressure, high-temperature regional metamorphism of igneous (granite) or sedimentary (shale/sandstone) protoliths. It usually signifies deep burial within the Earth's crust.
Uses & applications
Commonly used as crushed stone in construction, road base, and occasionally as structural stone or tiles if the banding is aesthetically pleasing.
Geological facts
Gneiss is among the oldest known rocks on Earth, with the Acasta Gneiss in Canada dating back approximately 4 billion years.
Field identification & locations
Identify by looking for distinct alternating bands of light and dark minerals. Found globally in ancient continental shields like the Appalachian Mountains or the Canadian Shield.
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