metamorphic
Gneiss
Gneiss (foliated metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Banded appearance with white/grey felsic and dark mafic layers; Luster: Dull to vitreous; Crystal structure: Coarse-grained, crystalline; Cleavage: Poorly defined, breaks along irregular surfaces.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded appearance with white/grey felsic and dark mafic layers; Luster: Dull to vitreous; Crystal structure: Coarse-grained, crystalline; Cleavage: Poorly defined, breaks along irregular surfaces.
Formation & geological history
Formed through high-grade regional metamorphism of igneous (granite) or sedimentary (shale/sandstone) protoliths under extreme heat and pressure. Most gneiss is hundreds of millions to billions of years old.
Uses & applications
Used in construction as crushed stone for road bases, as dimension stone for building facades, floor tiles, and gravestones. Larger specimens are used as decorative garden stones.
Geological facts
Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is approximately 4.03 billion years old. The term comes from the Middle High German verb 'gneist' meaning 'to spark'.
Field identification & locations
Identified in the field by its distinct 'gneissose banding' often showing wavy or contorted layers. Commonly found in the cores of mountain ranges and eroded crystalline shields. Look for alternating light and dark bands.
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