metamorphic
Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 (Mohs scale), Color: alternating dark and light bands (foliation), Luster: dull to pearly, Crystal structure: coarse-grained and crystalline, Cleavage: poor, Specific gravity: 2.6-2.9
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Physical properties
Hardness: 6-7 (Mohs scale), Color: alternating dark and light bands (foliation), Luster: dull to pearly, Crystal structure: coarse-grained and crystalline, Cleavage: poor, Specific gravity: 2.6-2.9
Formation & geological history
Formed by high-pressure and high-temperature regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale or sandstone). Often found in the cores of mountain ranges and ancient shield areas.
Uses & applications
Used as a dimension stone for flooring, facing on buildings, kitchen countertops, and as crushed stone for road construction and landscaping.
Geological facts
Gneiss is one of the oldest rocks found on Earth, with the Acasta Gneiss in Canada dated to approximately 4 billion years old. The term comes from the German word 'gneist', meaning sparks or glimmer.
Field identification & locations
Identified by 'gneissic banding' where minerals are segregated into distinct dark (biotite/amphibole) and light (quartz/feldspar) layers. Common in glaciated regions and riverbeds as water-worn cobbles.
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