metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Streaks of black, white, gray, and pink; Luster: Dull to vitreous; Structure: Foliated (gneissic banding); Cleavage: Poor; Specific gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Streaks of black, white, gray, and pink
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Streaks of black, white, gray, and pink; Luster: Dull to vitreous; Structure: Foliated (gneissic banding); 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 during mountain-building events.
Uses & applications
Used as crushed stone for road construction, as architectural veneer, flooring, gravestones, and occasionally as decorative garden stones.
Geological facts
The Acasta Gneiss in Canada is one of the oldest known intact crustal fragments on Earth, dated to approximately 4.03 billion years old.
Field identification & locations
Identify by distinct banding of minerals (foliation) where light and dark minerals are segregated. Common in large mountain ranges and ancient continental shields.
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