metamorphic
Gneiss (or Granite Gneiss)
Metamorphic rock rich in quartz, feldspar, and biotite/amphibole
Hardness around 6-7 on Mohs scale. Color ranges from dark gray and black with light pink or white banding. Dull to sub-vitreous luster, coarse-grained phaneritic or foliated texture.
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Physical properties
Hardness around 6-7 on Mohs scale. Color ranges from dark gray and black with light pink or white banding. Dull to sub-vitreous luster, coarse-grained phaneritic or foliated texture.
Formation & geological history
Formed by high-grade regional metamorphism of igneous rocks like granite, or sedimentary rocks like sandstone, under extreme heat and pressure during mountain-building events.
Uses & applications
Widely used as crushed stone for road base and construction aggregate, dimension stone for landscaping, building facades, and paving blocks.
Geological facts
Gneiss contains distinct banding (gneissic banding) formed by segregation of light and dark minerals under extreme metamorphic conditions. Some of the oldest rocks on Earth, such as the Acasta Gneiss in Canada (~4.0 billion years old), are gneiss.
Field identification & locations
Look for alternating bands or specks of light-colored quartz/feldspar and dark-colored mica or amphibole minerals. Found globally in continental shield regions and eroded mountain belts.
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