metamorphic
Gneiss (Granitic Gneiss)
Gneiss
Hardness: 6-7 on Mohs scale; Color: Gray, pink (orthoclase), and black (biotite/amphibole) banding; Luster: Dull to vitreous; Structure: Foliated with distinct mineral banding (gneissose banding); Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, pink (orthoclase), and black (biotite/amphibole) banding; Luster: Dull to vitreous; Structure: Foliated with distinct mineral banding (gneissose banding); Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks under extreme pressure and temperature. High-grade metamorphism typically occurs deep within the crust during mountain-building events.
Uses & applications
Used as a dimension stone for construction, flooring, gravestones, and as crushed stone for road aggregate and landscaping.
Geological facts
The Acasta Gneiss in Canada is one of the oldest known intact crustal fragments on Earth, dated to approximately 4.03 billion years old. Gneiss is often used to tell the story of ancient tectonic collisions.
Field identification & locations
Look for distinct light and dark mineral bands (layering) that are often contorted or wavy. It is widespread in continental shields and mountain ranges like the Appalachians or Alps. Collectors look for unique folding patterns.
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