metamorphic
Gneiss (Metamorphic Rock)
Gneiss (high-grade regional metamorphic rock)
Hardness: 6-7 on Mohs scale. Color: Gray, pink, white, and black banded appearance. Luster: Dull to vitreous. Structure: Foliated with distinct compositional banding (gneissose banding). Cleavage: Poor, usually breaks along uneven planes.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, pink, white, and black banded appearance
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Gray, pink, white, and black banded appearance. Luster: Dull to vitreous. Structure: Foliated with distinct compositional banding (gneissose banding). Cleavage: Poor, usually breaks along uneven planes.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths) under intense heat (over 600°C) and pressure. This specimen shows signs of fluvial erosion, suggesting it was transported by water.
Uses & applications
Used primarily as crushed stone for road construction and concrete aggregate. Select types are used as dimensional stone for countertops, flooring, and gravestones.
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 occurs because minerals segregate during intense pressure.
Field identification & locations
Identify by looking for 'gneissose banding'—alternating light and dark layers of minerals like quartz, feldspar, and mica. This piece shows water-worn rounding, common in riverbeds or glacial deposits.
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