metamorphic
Banded Gneiss
Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 (Mohs scale); Color: Banded layers of yellow-tan, dark grey, and green; Luster: Dull to sub-vitreous; Crystal structure: Coarse-grained phaneritic; Cleavage: Poor, usually breaks along foliation; Specific gravity: 2.6-2.9
- Hardness
- 6-7 (Mohs scale)
- Color
- Banded layers of yellow-tan, dark grey, and green
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Banded layers of yellow-tan, dark grey, and green; Luster: Dull to sub-vitreous; Crystal structure: Coarse-grained phaneritic; Cleavage: Poor, usually breaks along foliation; Specific gravity: 2.6-2.9
Formation & geological history
Formed through the high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under intense heat and pressure deep within the Earth's crust, leading to the segregation of minerals into bands (foliation).
Uses & applications
Used primarily as crushed stone for road construction, as ornamental stone in landscaping, or as building material for flooring, gravestones, and countertops.
Geological facts
Gneiss is some of the oldest rock on Earth, with the Acasta Gneiss in Canada being dated to approximately 4 billion years old. The term comes from the German word 'Gneis', meaning spark.
Field identification & locations
Identify in the field by looking for 'gneissose banding' consisting of alternating light and dark mineral layers. Commonly found in the cores of mountain ranges and ancient continental shields. Collectors look for sharp contrast in bands.
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