metamorphic
Gneiss (Banded Gneiss)
Gneiss
Hardness: 6-7 on Mohs scale; Color: Alternating light (quartz/feldspar) and dark (biotite/hornblende) bands; Luster: Dull to vitreous; Crystal Structure: Foliated with coarse grains; Cleavage: Poor; 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; Crystal Structure: Foliated with coarse grains; Cleavage: Poor; 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. Often represents some of the oldest crustal material on Earth, dating back billions of years.
Uses & applications
Used in construction as crushed stone for road bases, as dimension stone for building facades, countertops, and occasionally for ornamental landscaping and decorative masonry.
Geological facts
The Acasta Gneiss in Canada is one of the oldest known rock bodies on Earth, dated to approximately 4.03 billion years old. Gneiss does not split easily along its bands, unlike slate or schist.
Field identification & locations
Identify by distinct 'gneissic banding' (segregation of minerals into layers). Look for alternating stripes of color in riverbeds or mountain outcrops. It is common in the roots of mountain ranges and stable continental shields.
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