metamorphic
Banded Gneiss
Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 (Mohs scale), Color: Alternating light (gray/white) and dark (black/brown) bands, Luster: Dull to vitreous, Crystal Structure: Coarse-grained foliated, Cleavage: Poorly developed as it breaks along coarse grains, Specific Gravity: 2.6-2.9
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Alternating light (gray/white) and dark (black/brown) bands, Luster: Dull to vitreous, Crystal Structure: Coarse-grained foliated, Cleavage: Poorly developed as it breaks along coarse grains, Specific Gravity: 2.6-2.9
Formation & geological history
Formed by high-pressure, high-temperature metamorphism of granite or sedimentary rocks. The banding occurs due to recrystallization and migration of minerals into distinct layers during extreme tectonic events. Commonly found in ancient crystalline basements of continents.
Uses & applications
Used in construction as crushed stone for roads, as dimension stone for buildings, flooring, and kitchen countertops (often sold as 'Granite' in the trade).
Geological facts
Gneiss represents one of the highest grades of metamorphism. Some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada (approx. 4 billion years old), are gneiss specimens.
Field identification & locations
Identified by 'gneissose banding'—distinct layers of mineral concentrations. Found in mountainous or shield regions. For collectors, look for high-contrast banding and folding patterns.
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