metamorphic
Gneiss
Gneiss
Mohs hardness of 6-7, typically banded appearance with alternating light (quartz and feldspar) and dark (biotite and amphibole) mineral layers, vitreous to dull luster, granular to foliated crystal structure, specific gravity around 2.6-2.9.
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Physical properties
Mohs hardness of 6-7, typically banded appearance with alternating light (quartz and feldspar) and dark (biotite and amphibole) mineral layers, vitreous to dull luster, granular to foliated crystal structure, specific gravity around 2.6-2.9.
Formation & geological history
Formed by high-temperature and high-pressure metamorphic processes, usually originating from pre-existing igneous rocks (like granite) or sedimentary rocks (like shale or sandstone) deep within the Earth's crust during regional metamorphism.
Uses & applications
Commonly used in construction as crushed stone for road bases, building facades, and dimension stone for landscaping or countertops.
Geological facts
Gneiss contains some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada, which is dated at approximately 4.03 billion years old.
Field identification & locations
Easily identified in the field by its distinct alternating light and dark bands, known as gneissic banding. Found globally in ancient continental shields and mountain-building belts.
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