metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: typically banded with alternating dark (biotite, amphibole) and light (quartz, feldspar) layers; Luster: dull to vitreous; Structure: foliated with coarse grain size; Cleavage: poor/none (breaks irregularly).
- Hardness
- 6-7 on Mohs scale
- Luster
- dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: typically banded with alternating dark (biotite, amphibole) and light (quartz, feldspar) layers; Luster: dull to vitreous; Structure: foliated with coarse grain size; Cleavage: poor/none (breaks irregularly).
Formation & geological history
Forms through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (like granite or shale) under intense heat and pressure deep within the Earth's crust, often during mountain-building events.
Uses & applications
Commonly used as a crushed stone for road construction and concrete aggregate, and as a dimension stone for flooring, building facades, and decorative landscaping.
Geological facts
Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The term 'gneiss' comes from a German word meaning 'sparkling' or 'to glisten'.
Field identification & locations
Identify by looking for 'gneissose banding'—distinct parallel layers of different minerals. It is widely distributed in the ancient cores of continents (cratons) and in major mountain ranges like the Appalachians or Alps.
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