metamorphic
Gneiss (Banded Gneiss)
Gneiss
Hardness: 6-7 (Mohs); Color: Alternating light (gray, tan) and dark (black) bands; Luster: Dull to pearly; Crystal structure: Coarse-grained foliated; Cleavage: Poor, usually breaks along irregular surfaces.
- Hardness
- 6-7 (Mohs)
- Color
- Alternating light (gray, tan) and dark (black) bands
- Luster
- Dull to pearly
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Physical properties
Hardness: 6-7 (Mohs); Color: Alternating light (gray, tan) and dark (black) bands; Luster: Dull to pearly; Crystal structure: Coarse-grained foliated; Cleavage: Poor, usually breaks along irregular surfaces.
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (like granite or shale) subjected to intense heat and pressure deep within the crust.
Uses & applications
Used in construction as crushed stone for road bases, as dimension stone for counter tops and building facades, and as landscaping material.
Geological facts
Gneiss is some of the oldest rock on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The banding is caused by 'gneissic foliation' where minerals segregate into layers.
Field identification & locations
Identify by 'gneissic banding'—distinct layers of different colors. Commonly found in mountainous regions or stable continental interiors (cratons). Look for the separation of quartz/feldspar from dark mica/amphibole.
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