metamorphic
Gneiss
Gneiss (foliated metamorphic rock)
Hardness 6-7 on Mohs scale; color varies with dark biotiite/amphibole and light quartz/feldspar banding; vitreous to dull luster; distinct alternating dark and light compositional banding (gneissic foliation); non-cleavable whole rock with irregular, jagged fracture.
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Physical properties
Hardness 6-7 on Mohs scale; color varies with dark biotiite/amphibole and light quartz/feldspar banding; vitreous to dull luster; distinct alternating dark and light compositional banding (gneissic foliation); non-cleavable whole rock with irregular, jagged fracture.
Formation & geological history
Formed by high-grade regional metamorphism under extreme heat and pressure from pre-existing igneous (granite) or sedimentary (sandstone/shale) rocks deep within the Earth's crust.
Uses & applications
Used in construction as crushed stone for road bases, aggregate, decorative landscaping rock, building facing, and countertops.
Geological facts
Gneiss contains some of the oldest rocks found on Earth, such as the Acasta Gneiss in Canada, which is over 4 billion years old.
Field identification & locations
Identified by prominent alternating bands of light-colored silicate minerals and dark ferromagnesian minerals; common in mountain roots and ancient shield areas like the Canadian Shield.
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