metamorphic
Gneiss
Gneiss (Metamorphic rock composed primarily of Quartz, Feldspar, and Mica/Amphibole)
Hardness: 6-7 Mohs; Color: Pink, gray, white, and black banding; Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding (gneissose banding); Cleavage: None (fractures across bands)
- Hardness
- 6-7 Mohs
- Color
- Pink, gray, white, and black banding
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Pink, gray, white, and black banding; Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding (gneissose banding); Cleavage: None (fractures across bands)
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale) under extreme heat and pressure, often during continental collisions.
Uses & applications
Used in construction as crushed stone, dimension stone for flooring or countertops, and decorative landscaping.
Geological facts
Gneiss is one of the oldest known rocks on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The banding occurs because minerals segregate into layers during intense pressure.
Field identification & locations
Identify by looking for 'salty and pepper' banding or stripes. Common in mountainous regions like the Appalachians, Alps, or Canadian Shield. Look for alternating light (felsic) and dark (mafic) minerals.
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