metamorphic
Gneiss
Gneiss
Hardness: 6-7 (Mohs scale), Color: Banded grey, pink, brown, and green, Luster: Dull to vitreous, Crystal structure: Coarse-grained foliated, Cleavage: Poorly defined along bands, Specific Gravity: 2.6-2.9.
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Banded grey, pink, brown, and green, Luster: Dull to vitreous, Crystal structure: Coarse-grained foliated, Cleavage: Poorly defined along bands, Specific Gravity: 2.6-2.9.
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme pressure and temperature conditions deep in the Earth's crust. Common in ancient mountain belts and shields.
Uses & applications
Used as a dimension stone for architecture, flooring, countertops, and paving. Also used as crushed stone in construction and as decorative garden landscape rocks.
Geological facts
Gneiss displays a distinct banding called 'gneissose banding' results from the segregation of different minerals during metamorphism. Some of the oldest known rocks on Earth are Acasta Gneiss.
Field identification & locations
Identify by looking for alternating layers of light-colored (quartz/feldspar) and dark-colored (biotite/amphibole) minerals. Found in crystalline mountain ranges and glaciated plains.
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