metamorphic
Gneiss
Gneiss
Hardness of 6-7 on the Mohs scale; distinctive alternating light and dark banding (gneissic banding) composed of felsic minerals like quartz and feldspar alternating with mafic minerals like biotite and amphibole; vitreous to dull luster.
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Physical properties
Hardness of 6-7 on the Mohs scale; distinctive alternating light and dark banding (gneissic banding) composed of felsic minerals like quartz and feldspar alternating with mafic minerals like biotite and amphibole; vitreous to dull luster.
Formation & geological history
Formed by high-temperature and high-pressure metamorphic processes acting on pre-existing igneous or sedimentary rocks (such as granite or schist) deep within the Earth's crust, typically during regional mountain-building events.
Uses & applications
Used primarily in construction, as crushed stone for road aggregates, building facings, and occasionally as dimension stone for landscaping or countertops.
Geological facts
Gneiss contains some of the oldest rocks found on Earth, with certain formations dating back over 4 billion years, such as the Acasta Gneiss in Canada.
Field identification & locations
Easily identified in the field by its characteristic 'zebra-stripe' banding of light and dark minerals, which differentiates it from non-banded rocks like granite. Found globally in ancient shield areas and core of major mountain ranges.
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