metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Banded gray, white, and black; Luster: Dull to vitreous; Crystal structure: Foliated with distinct banding (gneissose banding); Cleavage: Poor; Specific gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Banded gray, white, and black
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded gray, white, and black; Luster: Dull to vitreous; Crystal structure: Foliated with distinct banding (gneissose banding); Cleavage: Poor; Specific gravity: 2.6-2.9.
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (like granite or shale) subjected to intense heat and pressure deep within the Earth's crust. It is often of Precambrian age, representing some of the oldest rocks on the planet.
Uses & applications
Used as a dimension stone for flooring, facing on buildings, and gravestones. It is also crushed for use in road construction, concrete aggregate, and railroad ballast.
Geological facts
Gneiss is one of the most common metamorphic rocks in the Earth's crust. Some of the oldest known rocks on Earth are the Acasta Gneiss in Canada, dated to approximately 4 billion years old.
Field identification & locations
Identify by looking for 'gneissose banding'—distinct layered bands of different minerals (usually light quartz/feldspar and dark biotite/amphibole). Common in mountainous regions and deeply eroded continental shields. Collect samples that clearly show the separation of mineral colors.
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