metamorphic
Gneiss
High-grade metamorphic rock (Gneiss)
Hardness: 6-7 (Mohs scale), Color: pinkish-red, gray, and black bands, Luster: dull to pearly, Crystal structure: phaneritic/foliated, Cleavage: poor, Specific gravity: 2.6-2.9
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Physical properties
Hardness: 6-7 (Mohs scale), Color: pinkish-red, gray, and black bands, Luster: dull to pearly, Crystal structure: phaneritic/foliated, Cleavage: poor, Specific gravity: 2.6-2.9
Formation & geological history
Formed by high-pressure and high-temperature regional metamorphism of pre-existing igneous or sedimentary rocks. This specimen likely dates from the Precambrian or Paleozoic era depending on its locale.
Uses & applications
Used as crushed stone for road construction, landscaping, and sometimes cut/polished for countertops or floor tiles.
Geological facts
Gneiss is one of the oldest known rock types on Earth; the Acasta Gneiss is approximately 4 billion years old. The term comes from the Middle High German verb 'gneist' (to spark), because the rock glitters.
Field identification & locations
Identify in the field by looking for 'gneissose banding'—distinct light and dark layers of minerals like quartz, feldspar, and biotite. Common in mountainous regions or glacial deposits.
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