metamorphic
Gneiss
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Dark grey, black, and tan bands; Luster: Dull to slightly vitreous; Structure: Foliated with distinct compositional banding (gneissose banding); Texture: Medium to coarse-grained.
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey, black, and tan bands
- Luster
- Dull to slightly vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey, black, and tan bands; Luster: Dull to slightly vitreous; Structure: Foliated with distinct compositional banding (gneissose banding); Texture: Medium to coarse-grained.
Formation & geological history
Formed under high-temperature and high-pressure conditions (regional metamorphism) from volcanic or sedimentary protoliths. These rocks are often billions of years old, dating back to the Precambrian era, and form deep within the Earth's crust at convergent plate boundaries.
Uses & applications
Used as a crushed stone in construction, road base, and as decorative landscaping stones. Higher quality slabs are used for flooring, gravestones, and countertops.
Geological facts
Gneiss includes some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada, which is approximately 4.03 billion years old. The bands form because minerals segregate into layers during intense pressure.
Field identification & locations
Identify in the field by looking for 'gneissose banding'—distinct light and dark layers. Unlike schist, it does not split easily along planes. Found in the roots of mountain ranges and ancient continental shields.
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