metamorphic
Gneiss (specifically Granitic Gneiss)
Gneiss
Hardness: 6-7 on Mohs scale; Color: Pink, white, and dark grey/green bands; Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding (gneissose banding); Cleavage: Generally poor, breaks along foliations.
- Hardness
- 6-7 on Mohs scale
- Color
- Pink, white, and dark grey/green bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pink, white, and dark grey/green bands; Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding (gneissose banding); Cleavage: Generally poor, breaks along foliations.
Formation & geological history
Formed through high-grade regional metamorphism of igneous rocks (granite) or sedimentary rocks under extreme pressure and temperature conditions. This specimen shows water-worn rounding, likely from a fluvial or glacial environment.
Uses & applications
Used primarily as crushed stone for road construction, concrete aggregate, and as dimension stone for building facades, flooring, and gravestones due to its durability.
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 name comes from a German word meaning 'sparkling'.
Field identification & locations
Identify in the field by looking for 'banding' where minerals have separated into layers of light (quartz/feldspar) and dark (biotite/amphibole). Common in mountain belts and Precambrian shield areas.
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