metamorphic
Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 (Mohs scale), Color: Banded or speckled gray, brown, and tan, Luster: Dull to vitreous, Crystal structure: Coarse-grained foliated, Specific gravity: 2.6-2.9
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Banded or speckled gray, brown, and tan, Luster: Dull to vitreous, Crystal structure: Coarse-grained foliated, Specific gravity: 2.6-2.9
Formation & geological history
Formed by high-pressure and high-temperature metamorphosis of igneous (granite) or sedimentary rocks. This specimen shows characteristic phaneritic textures and mineral segregation.
Uses & applications
Used primarily as crushed stone for road construction, building stone, architectural facing, and sometimes as polished decorative countertops.
Geological facts
Gneiss is some of the oldest rock on Earth, with the Acasta Gneiss in Canada dated at 4.03 billion years old. The term 'gneiss' comes from a Middle High German verb meaning 'to spark' or 'glitter'.
Field identification & locations
Identified in the field by its 'gneissic banding'—alternating layers of light and dark minerals. Common in the roots of mountain ranges and shield areas like the Canadian Shield or the Appalachians.
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