metamorphic
Banded Gneiss
Gneiss (high-grade metamorphic silicate rock)
Hardness: 6.5-7 (Mohs scale); Color: Black, gray, and tan bands; Luster: Dull to sub-vitreous; Structure: Foliated with distinct bands (gneissose banding); Cleavage: None, usually breaks with irregular fracture.
- Hardness
- 6
- Color
- Black, gray, and tan bands
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6.5-7 (Mohs scale); Color: Black, gray, and tan bands; Luster: Dull to sub-vitreous; Structure: Foliated with distinct bands (gneissose banding); Cleavage: None, usually breaks with irregular fracture.
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary protoliths under extreme heat and pressure, typically during mountain-building events (orogenies). It is most commonly found in Precambrian continental shields.
Uses & applications
Used primarily as crushed stone for road construction and aggregate, also used as dimension stone for architectural facings, counter tops, and decorative landscaping rock.
Geological facts
Gneiss is some of the oldest rock found on Earth, with the Acasta Gneiss in Canada dated to approximately 4 billion years old. The term 'gneiss' comes from an Old High German verb meaning 'to spark' because the rock glitters.
Field identification & locations
Identify in the field by looking for 'gneissose banding,' which consists of alternating layers of light-colored (felsic) and dark-colored (mafic) minerals. Common minerals include quartz, feldspar, and mica.
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