metamorphic
Gneiss
Gneiss
Hardness: 6-7 (Mohs scale); Color: Bands of light (quartz/feldspar) and dark (mica/amphibole); Luster: Vitreous to dull; Structure: Foliated (gneissose banding); Cleavage: Poorly defined; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 (Mohs scale)
- Color
- Bands of light (quartz/feldspar) and dark (mica/amphibole)
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Bands of light (quartz/feldspar) and dark (mica/amphibole); Luster: Vitreous to dull; Structure: Foliated (gneissose banding); Cleavage: Poorly defined; Specific Gravity: 2.6-2.9
Formation & geological history
Forms through high-grade regional metamorphism of igneous or sedimentary rocks under intense heat and pressure. It is often found in the cores of mountain ranges and ancient continental shields (crystalline basement rock).
Uses & applications
Used as a crushed stone for road construction, concrete aggregate, and railroad ballast. High-quality slabs are used for countertops, flooring, and decorative building stone.
Geological facts
Gneiss is one of the oldest known rocks on Earth; the Acasta Gneiss in Canada is dated at approximately 4.03 billion years old.
Field identification & locations
Identify by distinct light and dark mineral bands (foliation). Unlike schist, it does not split easily along planes. Often found in glaciated riverbeds or near mountain outcrops.
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