metamorphic
Banded Gneiss
Gneiss (Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Alternating light (quartz/feldspar) and dark (biotite/hornblende) bands; Luster: Dull to vitreous; Structure: Foliated; Cleavage: Generally poor, breaks along blocky planes; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Alternating light (quartz/feldspar) and dark (biotite/hornblende) bands; Luster: Dull to vitreous; Structure: Foliated; Cleavage: Generally poor, breaks along blocky planes; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under intense heat and pressure deep within the Earth's crust, often during mountain-building events (orogenies).
Uses & applications
Used primarily as a decorative stone in architecture, countertops, flooring, and as crushed stone for road construction and landscaping.
Geological facts
Gneiss is among the oldest known rocks on Earth, with some specimens like the Acasta Gneiss dating back over 4 billion years. The distinct banding is known as 'gneissose banding'.
Field identification & locations
Identify in the field by its characteristic 'zebra-stripe' layering and coarse grain size. Often found in glaciated regions as rounded river or beach pebbles like the one pictured.
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