metamorphic
Gneiss (Granitic Gneiss)
Gneiss (composed mainly of quartz, K-feldspar, plagioclase, and biotite)
Hardness 6-7 on Mohs scale. Color ranges from grey to pinkish-red and black. Features characteristic compositional banding (gneissic foliation). Luster is dull to vitreous. Granular texture with medium to coarse grains.
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Physical properties
Hardness 6-7 on Mohs scale. Color ranges from grey to pinkish-red and black. Features characteristic compositional banding (gneissic foliation). Luster is dull to vitreous. Granular texture with medium to coarse grains.
Formation & geological history
Formed under high-grade regional metamorphism (high temperatures and pressures) acting upon igneous rocks like granite or sedimentary rocks like sandstone. Typically Precambrian to Paleozoic in age.
Uses & applications
Widely used as crushed stone for road construction, railroad ballast, building facings, landscape rocks, and dimension stone for countertops and monuments.
Geological facts
The Acasta Gneiss in the Northwest Territories, Canada, is one of the oldest known intact crustal rocks on Earth, dated to over 4 billion years old.
Field identification & locations
Identified by distinct light (quartz/feldspar) and dark (biotite/amphibole) alternating bands. Commonly found in mountainous regions and shield areas globally. Extremely common and durable stone.
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