metamorphic
Gneiss
Gneiss
Hardness 6-7 on Mohs scale. Gray, white, and dark banded color scheme. Vitreous to dull luster, medium to coarse-grained texture with distinct foliation (gneissic banding). Composed primarily of quartz, feldspar, and biotite mica or amphibole.
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Physical properties
Hardness 6-7 on Mohs scale. Gray, white, and dark banded color scheme. Vitreous to dull luster, medium to coarse-grained texture with distinct foliation (gneissic banding). Composed primarily of quartz, feldspar, and biotite mica or amphibole.
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing sedimentary (like shale or sandstone) or igneous rocks (like granite) under high temperature and high pressure conditions deep within continental crust during mountain-building events.
Uses & applications
Used widely as crushed stone in road construction, concrete aggregate, and dimension stone for building facades, paving, counter tops, and landscape curbing.
Geological facts
The oldest known intact crustal rocks on Earth are the Acasta Gneisses in northern Canada, dated to approximately 4.03 billion years old.
Field identification & locations
Identify by alternating light (quartz and feldspar) and dark (biotite, hornblende) mineral bands that give it a striped appearance without easy splitting. Common in mountainous regions and exposed Precambrian shields globally.
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