metamorphic
Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 (Mohs scale), Color: Banded grey, white, and black, 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, white, and black, 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-pressure, high-temperature metamorphosis of granite or sedimentary rocks (protoliths). It represents a high grade of metamorphism where minerals segregate into bands.
Uses & applications
Used in flooring, ornamental stones, gravestones, facing stones on buildings, and as crushed stone for road construction and railroad ballast.
Geological facts
Gneiss is some of the oldest rock on Earth, with the Acasta Gneiss in Canada being dated to approximately 4 billion years old. The bands are caused by the separation of dark minerals like biotite and light minerals like quartz or feldspar.
Field identification & locations
Identified by distinct light and dark 'gneissic banding' or foliation. It is found in the deeply eroded roots of mountain ranges and in Precambrian shield areas. Look for parallel mineral alignment that does not split easily like schist.
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