metamorphic
Gneiss
Orthogneiss / Paragneiss
Hardness: 6-7 on Mohs scale, Color: Varies with alternating light and dark bands (gneissic banding), Luster: Dull to vitreous, Crystal structure: Foliated, granular texture, Specific Gravity: 2.6-2.9
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Physical properties
Hardness: 6-7 on Mohs scale, Color: Varies with alternating light and dark bands (gneissic banding), Luster: Dull to vitreous, Crystal structure: Foliated, granular texture, Specific Gravity: 2.6-2.9
Formation & geological history
Formed by high-grade regional metamorphism, where pre-existing rocks (igneous or sedimentary) are subjected to intense heat and directed pressure deep within the Earth's crust, typically during mountain-building events.
Uses & applications
Used primarily in construction, as crushed stone for road aggregates, and occasionally as dimension stone for building facades and paving.
Geological facts
Gneiss contains the oldest known rock material on Earth, with zircon crystals found in gneiss units in Western Australia dating back about 4.4 billion years.
Field identification & locations
Identified easily in the field by its distinct alternating light-colored bands of quartz and feldspar and dark-colored bands of biotite or amphibole. Found globally in ancient shields and core areas of mountain ranges.
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