metamorphic
Gneiss
Gneissic Metamorphic Rock
Hardness: 6-7 on Mohs scale; Color: Gray, black, and white bands; Luster: Vitreous to dull; Structure: Gneissic banding (foliated); Cleavage: Poor; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, black, and white bands
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, black, and white bands; Luster: Vitreous to dull; Structure: Gneissic banding (foliated); Cleavage: Poor; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale) under intense heat and pressure deep within the crust.
Uses & applications
Used as crushed stone for road construction, paving stones, facing stone on buildings, and sometimes as ornamental kitchen countertops (often sold as 'granite').
Geological facts
Gneiss is among the oldest known rocks on Earth, with some specimens from the Acasta Gneiss complex in Canada dating back over 4 billion years.
Field identification & locations
Identify in the field by looking for 'gneissic banding', where minerals are segregated into distinct dark and light colored layers. Commonly found in the roots of mountain ranges and shield areas.
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