metamorphic
Gneiss
Metamorphic Gneiss (High-grade regional metamorphism)
Hardness: 6-7 Mohs; Color: Banded grey, black, and white; Luster: Dull to slightly pearly; Crystal structure: Coarse-grained foliated; Cleavage: Poor; Specific gravity: 2.6-3.0
- Hardness
- 6-7 Mohs
- Color
- Banded grey, black, and white
- Luster
- Dull to slightly pearly
Identified More metamorphic →
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Physical properties
Hardness: 6-7 Mohs; Color: Banded grey, black, and white; Luster: Dull to slightly pearly; Crystal structure: Coarse-grained foliated; Cleavage: Poor; Specific gravity: 2.6-3.0
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary rocks under extreme pressure and temperature. The bands represent different mineral layers segregated by heat and pressure.
Uses & applications
Commonly used in construction as crushed stone for road bases, as a building stone for facades, and occasionally as ornamental landscaping stones.
Geological facts
Gneiss is some of the oldest rock on Earth, with the Acasta Gneiss in Canada dating back over 4 billion years. The distinctive 'gneissic banding' is caused by mineral segregation.
Field identification & locations
Identified by its distinctive bands of light and dark minerals (foliation) and lack of distinct cleavage compared to schist. Found globally in the cores of mountain ranges and ancient continental shields.
