metamorphic
Gneiss
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Gray, white, and black banded; Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding (gneissic banding); Texture: Coarse-grained.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, white, and black banded
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, white, and black banded; Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding (gneissic banding); Texture: Coarse-grained.
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous or sedimentary rocks. This specimen shows significant rounding from water erosion (beach/river tumble). Geological age can range from Precambrian to Cenozoic depending on location.
Uses & applications
Used as a dimension stone for flooring and building facades, crushed stone for road construction, and occasionally for landscaping or as decorative garden stones.
Geological facts
Gneiss is one of the oldest known rock types on Earth, with some specimens like the Acasta Gneiss dating back over 4 billion years. The light bands are typically quartz/feldspar and the dark bands are biotite/amphibole.
Field identification & locations
Identified by 'Gneissic banding' where minerals are segregated into distinct layers. Commonly found in mountainous regions or shield areas. Look for the characteristic striped or banded appearance in hard, crystalline rock.
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