metamorphic
Banded Gneiss
Gneiss (High-grade regional metamorphic rock)
Hardness: 6-7 (Mohs); Color: Dark grey to brown with tan or yellowish bands; Luster: Dull to resinous (when wet); Structure: Foliated with distinct compositional banding (gneissose banding); Cleavage: Poor.
- Hardness
- 6-7 (Mohs)
- Color
- Dark grey to brown with tan or yellowish bands
- Luster
- Dull to resinous (when wet)
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Physical properties
Hardness: 6-7 (Mohs); Color: Dark grey to brown with tan or yellowish bands; Luster: Dull to resinous (when wet); Structure: Foliated with distinct compositional banding (gneissose banding); Cleavage: Poor.
Formation & geological history
Formed through high-grade regional metamorphism of igneous (protolith often granite or diorite) or sedimentary rocks under extreme pressure and temperature, typically during mountain-building events. Ages vary from hundreds of millions to billions of years.
Uses & applications
Used as crushed stone for road construction, decorative aggregate, building stone, and occasionally for landscaping or low-cost lapidary practice.
Geological facts
Gneiss contains some of the oldest known rock formations on Earth, such as the Acasta Gneiss in Canada, which is approximately 4 billion years old.
Field identification & locations
Identify in the field by looking for alternating light and dark mineral bands. It is extremely common in mountainous regions or glacial drift. Collectors look for high-contrast banding patterns.
