metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: typically banded with alternating light (quartz/feldspar) and dark (biotite/amphibole) layers; Luster: dull to vitreous; Crystal structure: phaneritic, foliated (gneissose); Cleavage: poor; Specific gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Luster
- dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: typically banded with alternating light (quartz/feldspar) and dark (biotite/amphibole) layers; Luster: dull to vitreous; Crystal structure: phaneritic, foliated (gneissose); Cleavage: poor; Specific gravity: 2.6-2.9.
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale) at high temperatures and pressures. Often Precambrian in age but found across various geological eras.
Uses & applications
Used as a crushed stone in construction, road building, and as dimension stone for kitchen countertops, flooring, and gravestones. It is also a popular collector specimen for its unique banding.
Geological facts
Gneiss is one of the oldest rocks on Earth, with the Acasta Gneiss in Canada being dated to approximately 4 billion years old. The term comes from the Middle High German verb 'gneist' (to spark), because the rock glitters.
Field identification & locations
Identified by its characteristic 'gneissic banding' where minerals are segregated into distinct layers. Commonly found in mountainous regions or glacial deposits. Collectors look for sharp, high-contrast color bands.
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