metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Gray, black, and white bands; Luster: Variable, dull to pearly; Crystal structure: Foliated (banded); Cleavage: Poor; Specific Gravity: 2.6-3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, black, and white bands
- Luster
- Variable, dull to pearly
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, black, and white bands; Luster: Variable, dull to pearly; Crystal structure: Foliated (banded); Cleavage: Poor; Specific Gravity: 2.6-3.0.
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing sedimentary rocks like shale or igneous rocks like granite. Under high pressure and temperature, minerals recrystallize and segregate into bands. Carboniferous to Precambrian ages are common.
Uses & applications
Commonly used as crushed stone in road construction and landscaping, or as dimension stone for architectural facades, flooring, and gravestones.
Geological facts
Gneiss contains some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada, which is approximately 4.03 billion years old.
Field identification & locations
Identified by distinct light and dark mineral banding (gneissose banding). Found in the crystalline cores of many mountain ranges and shield areas globally. Look for alternating layers of quartz/feldspar and biotite/hornblende.
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