metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Grey, white, black, or pinkish with banded patterns; Luster: Dull to vitreous; Crystal structure: Coarse-grained, foliated; Cleavage: Poor, typically breaks along grain boundaries; Specific gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Grey, white, black, or pinkish with banded patterns
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Grey, white, black, or pinkish with banded patterns; Luster: Dull to vitreous; Crystal structure: Coarse-grained, foliated; Cleavage: Poor, typically breaks along grain boundaries; Specific gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks like granite or sedimentary rocks like shale. It is usually formed deep within the Earth's crust under intense heat and pressure, often found in the roots of mountain ranges.
Uses & applications
Used primarily as decorative stone, crushed stone for road construction, paving material, and occasionally for countertops or architectural facings due to its durability and unique banding.
Geological facts
Gneiss is some of the oldest rock on Earth, with the Acasta Gneiss in Canada dated to approximately 4 billion years old. The term origins is from the German 'Gneis', meaning 'to spark'.
Field identification & locations
Identify in the field by its distinct alternating layers or bands of light and dark minerals (foliation). It is found globally in Precambrian shields and metamorphic terranes such as the Himalayas or Alps.
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