metamorphic
Banded Gneiss
Gneiss (Metamorphic high-grade rock)
Hardness: 6-7 on Mohs scale; Color: Alternating dark (mafic) and light (felsic) bands of black, grey, tan, and reddish-orange; Luster: Dull to sub-vitreous; Structure: Foliated / Gneissose banding; Cleavage: Poorly defined, usually breaks across bands.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Alternating dark (mafic) and light (felsic) bands of black, grey, tan, and reddish-orange; Luster: Dull to sub-vitreous; Structure: Foliated / Gneissose banding; Cleavage: Poorly defined, usually breaks across bands.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous (like granite) or sedimentary rocks (like shale) at high temperatures and pressures. This processes facilitates the segregation of minerals into distinct layers. Age can range from hundreds of millions to billions of years (Precambrian to Cenozoic).
Uses & applications
Commonly used as a decorative stone in architecture, countertops, flooring, and as crushed stone or aggregate in road construction and landscaping.
Geological facts
Gneiss is some of the oldest rock found on Earth, with some specimens in Northern Canada dated to over 4 billion years old. The term 'Gneiss' comes from an old German word meaning 'sparkling' or 'to glisten.'
Field identification & locations
Identify by looking for distinct 'gneissose banding'—parallel layers of different minerals. Unlike schist, it does not split easily along planes. Often found in mountainous regions or eroded riverbeds. Collectors look for high-contrast banding and vibrant colors.
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