metamorphic
Gneiss (specifically Banded Gneiss)
Gneiss
Hardness: 6-7 (Mohs scale); Color: Gray, white, and dark bands; Luster: Dull to pearly; Crystal structure: Foliated (banded); Cleavage: Poorly defined; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 (Mohs scale)
- Color
- Gray, white, and dark bands
- Luster
- Dull to pearly
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Gray, white, and dark bands; Luster: Dull to pearly; Crystal structure: Foliated (banded); Cleavage: Poorly defined; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary rocks (protoliths) under intense heat and pressure deep within the Earth's crust, often during mountain-building events.
Uses & applications
Used in construction as crushed stone for road bases, as ornamental stone in landscaping (boulders), and as dimension stone for countertops and building facades.
Geological facts
The Acasta Gneiss in Canada is one of the oldest known intact crustal fragments on Earth, dated to approximately 4.03 billion years old.
Field identification & locations
Identify in the field by looking for 'gneissose banding'—distinct light and dark parallel layers where minerals like quartz/feldspar and mica/amphibole have segregated. Common in crystalline basement complexes like the Canadian Shield or the Appalachians.
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