metamorphic
Gneiss
High-grade foliated metamorphic rock
Hardness: 6-7 (Mohs scale), Color: Alternating dark and light bands (usually white/gray and black), Luster: Dull to pearly/vitreous depending on mineral content, Structure: Gneissic banding (foliated), Specific Gravity: 2.6-2.9
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Alternating dark and light bands (usually white/gray and black), Luster: Dull to pearly/vitreous depending on mineral content, Structure: Gneissic banding (foliated), Specific Gravity: 2.6-2.9
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme heat and pressure. It often dates back to the Precambrian era and occurs in deep orogenic (mountain-building) belts.
Uses & applications
Used extensively in the construction industry as crushed stone for road bases and as dimension stone for architectural facades, countertops, and decorative landscaping.
Geological facts
Gneiss is some of the oldest rock found on Earth, with the Acasta Gneiss in Canada dated to approximately 4 billion years old. The banding occurs due to chemical segregation of minerals during metamorphism.
Field identification & locations
Identify in the field by looking for distinct 'gneissic banding', where minerals like quartz/feldspar (light) and biotite/hornblende (dark) form layers. Commonly found in shield areas of continents and core mountain ranges like the Appalachians or Alps.
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