metamorphic
Gneiss
Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Gray, black, and white banded layers; Luster: Dull to pearly; Crystal structure: Crystalline with visible mineral grains; Cleavage: Poor, but splits along bands; Specific gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, black, and white banded layers
- Luster
- Dull to pearly
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, black, and white banded layers; Luster: Dull to pearly; Crystal structure: Crystalline with visible mineral grains; Cleavage: Poor, but splits along bands; Specific gravity: 2.6-2.9
Formation & geological history
Formed through high-pressure, high-temperature metamorphism of granite or sedimentary rocks. It undergoes significant mineral recrystallization and segregation into bands. Common in Precambrian shields and deeply buried mountain roots.
Uses & applications
Used primarily as crushed stone for road construction and concrete aggregate. High-quality slabs are used for countertops, building facades, and ornamental landscaping stones.
Geological facts
Gneiss contains some of the oldest rocks on Earth, such as the Acasta Gneiss in Canada, which is approximately 4 billion years old. The term comes from the Middle High German word 'gneist', meaning 'spark'.
Field identification & locations
Identify by 'gneissose banding', which is the alternation of light-colored (quartz/feldspar) and dark-colored (biotite/amphibole) minerals. Found in various mountain ranges world-wide. Key for mapping crustal deformation.
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