metamorphic
Gneiss
Gneiss (Metamorphic Rock)
Hardness: 6-7 on the Mohs scale. Color: Alternating light gray, white, dark gray, and iron-stained brownish-yellow bands. Luster: Dull to pearly/sub-vitreous. Texture: Medium- to coarse-grained with distinctive gneissic banding (foliation).
- Hardness
- 6-7 on the Mohs scale
- Luster
- Dull to pearly/sub-vitreous
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Physical properties
Hardness: 6-7 on the Mohs scale. Color: Alternating light gray, white, dark gray, and iron-stained brownish-yellow bands. Luster: Dull to pearly/sub-vitreous. Texture: Medium- to coarse-grained with distinctive gneissic banding (foliation).
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing sedimentary or igneous rocks (such as granite or shale) subjected to high temperatures and pressures deep within the Earth's crust.
Uses & applications
Widely used in construction as crushed stone for road bases and concrete aggregate, as well as dimension stone for building facades, counter tops, flagging, and landscaping.
Geological facts
Gneissic banding forms because minerals segregate under intense directional pressure—felsic minerals like quartz and feldspar form light bands, while mafic minerals like biotite and amphibole form dark bands.
Field identification & locations
Identified in the field by prominent, alternating light and dark mineral bands that do not split easily along the mineral layers, unlike schist. Commonly found in ancient shield regions and eroded mountain belts globally.
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