metamorphic
Granite Gneiss
Gneiss (Derived from Granite protolith)
Hardness: 6-7 on the Mohs scale. Color: Mottled pink, white, gray, and dark green/black (due to alternating bands of quartz, feldspar, and biotite/amphibole). Luster: Vitreous to sub-vitreous. Crystal structure: Foliated, banded texture with interlocking mineral grains. Cleavage: Poor, breaks across mineral bands.
- Hardness
- 6-7 on the Mohs scale
- Luster
- Vitreous to sub-vitreous
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Physical properties
Hardness: 6-7 on the Mohs scale. Color: Mottled pink, white, gray, and dark green/black (due to alternating bands of quartz, feldspar, and biotite/amphibole). Luster: Vitreous to sub-vitreous. Crystal structure: Foliated, banded texture with interlocking mineral grains. Cleavage: Poor, breaks across mineral bands.
Formation & geological history
Formed through regional metamorphism of an igneous protolith (granite or granodiorite) under extreme heat and pressure deep within the Earth's crust, typically during mountain-building (orogenic) events spanning hundreds of millions of years (Precambrian to Phanerozoic).
Uses & applications
Used widely as dimension stone in architecture, exterior cladding, floor tiles, and gravestones, as well as crushed stone for road construction.
Geological facts
Gneiss contains some of the oldest rocks found on Earth, with certain formations dating back over 4 billion years. The characteristic alternating light and dark bands are known as gneissic banding.
Field identification & locations
Easily identified by its distinctive banded or striped appearance, contrasting pink/white felsic minerals with dark mafic minerals. Commonly found in ancient continental shields, glacial erratics, and stream beds worldwide.
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