metamorphic
Gneiss
Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 (Mohs scale); Color: Banded appearance of light colors (quartz, feldspar) and dark colors (biotite, amphibole); Luster: Dull to pearly; Crystal structure: Foliated with distinct banding (gneissose banding); Specific gravity: 2.6-2.9.
- Hardness
- 6-7 (Mohs scale)
- Luster
- Dull to pearly
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Banded appearance of light colors (quartz, feldspar) and dark colors (biotite, amphibole); Luster: Dull to pearly; Crystal structure: Foliated with distinct banding (gneissose banding); Specific gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary protoliths under extreme heat and pressure, typically found in the roots of mountain ranges. Dates from the Precambrian to more recent orogenic events.
Uses & applications
Used as crushed stone in construction, road aggregate, and sometimes as dimension stone for landscaping or countertops (often sold as 'granite').
Geological facts
Gneiss contains minerals that have been recrystallized and segregated into distinct layers. Some of the oldest rocks on Earth, like the Acasta Gneiss (over 4 billion years old), are types of gneiss.
Field identification & locations
Identify by looking for clear alternating bands of light and dark minerals. It will not split easily along planes like schist. Commonly found in shield areas of continents and core mountain belts.
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