metamorphic
Gneiss
Gneiss
Hardness: 6-7 on the Mohs scale; Color: alternating light (quartz, feldspar) and dark (biotite, hornblende) bands; Luster: dull to vitreous; Texture: foliated with distinct gneissic banding; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on the Mohs scale
- Luster
- dull to vitreous
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Physical properties
Hardness: 6-7 on the Mohs scale; Color: alternating light (quartz, feldspar) and dark (biotite, hornblende) bands; Luster: dull to vitreous; Texture: foliated with distinct gneissic banding; Specific Gravity: 2.6-2.9
Formation & geological history
Formed by high-grade regional metamorphism of igneous or sedimentary rocks (such as granite, shale, or sandstone) under extreme temperature and pressure deep within Earth's crust.
Uses & applications
Widely used as crushed stone in construction, road base, aggregate, and as decorative dimension stone, countertops, and landscaping rock.
Geological facts
The oldest known intact crustal rocks on Earth are the Acasta Gneiss in Northwest Territories, Canada, dated to approximately 4.03 billion years old.
Field identification & locations
Identify by its characteristic compositional banding of alternating light and dark minerals without prominent mineral cleavage planes like schist. Commonly found in exposed Precambrian shields and mountainous metamorphic terrains globally.
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