metamorphic
Banded Gneiss
Gneiss (Felsic to Mafic Metamorphic Rock)
Hardness 6-7 on Mohs scale. Distinctive alternating light (quartz/feldspar) and dark (biotite/amphibole) bands (gneissic banding). Waxy to dull luster on weathered surfaces, crystalline sub-vitreous on fresh surfaces.
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Physical properties
Hardness 6-7 on Mohs scale. Distinctive alternating light (quartz/feldspar) and dark (biotite/amphibole) bands (gneissic banding). Waxy to dull luster on weathered surfaces, crystalline sub-vitreous on fresh surfaces.
Formation & geological history
Formed through high-grade regional metamorphism under extreme heat and pressure, typically transforming pre-existing igneous rocks (like granite) or sedimentary rocks (like sandstone/shale).
Uses & applications
Used as crushed stone in construction, road base, dimension stone for buildings and countertops, landscaping, and decorative rock collections.
Geological facts
Gneiss is one of the oldest rock types on Earth, with some specimens like the Acasta Gneiss in Canada dating back over 4 billion years.
Field identification & locations
Look for pronounced alternating dark and light mineral bands with visible coarse mineral grains. Commonly found in shield regions and eroded mountain belts globally.
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