metamorphic
Gneiss
Gneiss (High-grade metamorphic rock typically composed of Quartz, Feldspar, and Mica/Amphibole)
Hardness: 6-7 on Mohs scale; Color: Pink, gray, white, and black; Luster: Vitreous to dull; Crystal structure: Crystalline with diagnostic foliation called 'gneissose banding'; Cleavage: Non-existent in the rock mass, though individual minerals may show it; Specific Gravity: 2.6 - 2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Pink, gray, white, and black
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pink, gray, white, and black; Luster: Vitreous to dull; Crystal structure: Crystalline with diagnostic foliation called 'gneissose banding'; Cleavage: Non-existent in the rock mass, though individual minerals may show it; Specific Gravity: 2.6 - 2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths) under extreme pressure and temperature conditions (over 600°C), typically during mountain-building events (orogenies). This specimen shows glacial rounding, suggesting it was transport by ice during the Pleistocene.
Uses & applications
Used primarily in the construction industry as crushed stone for road bases and concrete aggregate. High-quality slabs are used for flooring, countertops (often marketed as 'granite'), curbing, and as facing stones for buildings.
Geological facts
Gneiss is among the oldest known rocks on Earth; the Acasta Gneiss in Canada is dated at approximately 4.03 billion years old. The term 'gneiss' comes from an old German verb meaning 'to spark' or 'to glisten.'
Field identification & locations
Identify in the field by looking for 'segregation banding'—alternating layers of light-colored (felsic) and dark-colored (mafic) minerals. Unlike schist, it does not split easily along planes. Commonly found in the cores of mountain ranges and ancient continental shields like the Canadian Shield or the Appalachian Mountains.
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