metamorphic
Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 (Mohs scale); Color: Banded grey, white, and tan; Luster: Vitreous to dull; Crystal structure: Coarse-grained foliated texture (gneissose banding); Cleavage: Poor cleavage but splits along bands; Specific gravity: 2.6-2.9.
- Hardness
- 6-7 (Mohs scale)
- Color
- Banded grey, white, and tan
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Banded grey, white, and tan; Luster: Vitreous to dull; Crystal structure: Coarse-grained foliated texture (gneissose banding); Cleavage: Poor cleavage but splits along bands; Specific gravity: 2.6-2.9.
Formation & geological history
Formed from regional metamorphism of igneous (granite) or sedimentary (shale/sandstone) protoliths under extreme temperature and pressure deep within the Earth's crust, often during mountain-building events.
Uses & applications
Primarily used as a crushed stone in road construction, concrete aggregate, and as decorative dimension stone for building facades, countertops, and flooring due to its durability.
Geological facts
Gneiss contains some of the oldest known rock on Earth, such as the Acasta Gneiss in Canada which is roughly 4 billion years old. The distinctive banding is called 'gneissic banding'.
Field identification & locations
Identify by the presence of alternating light-colored (felsic) and dark-colored (mafic) bands. It is commonly found in the Precambrian shields of continents like the Canadian Shield or the Scandinavian Shield.
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