metamorphic
Gneiss
Gneiss (SiO2-rich metamorphic rock)
Hardness 6-7 on Mohs scale; interlocking crystalline texture; vitreous to dull luster; distinctive light and dark compositional banding (gneissic foliation); specific gravity around 2.6-2.9.
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Physical properties
Hardness 6-7 on Mohs scale; interlocking crystalline texture; vitreous to dull luster; distinctive light and dark compositional banding (gneissic foliation); specific gravity around 2.6-2.9.
Formation & geological history
Forms by high-grade regional metamorphism under extreme heat and pressure, often transforming pre-existing igneous (like granite) or sedimentary rocks deep within continental crust.
Uses & applications
Used as crushed stone in construction, road base, aggregate, and as a durable decorative dimension stone for countertops and building facades.
Geological facts
The Acasta Gneiss in the Northwest Territories of Canada is one of the oldest known intact crustal rocks on Earth, dated to approximately 4.02 billion years old.
Field identification & locations
Identified by its visible parallel light (feldspar, quartz) and dark (biotite, amphibole) mineral bands. Commonly found in exposed Precambrian shield areas and mountain cores worldwide.
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