metamorphic
Gneiss
Gneiss (High-grade Regional Metamorphism)
Hardness: 6-7. Color: Banded grey, black, pink, and white. Luster: Vitreous to dull. Structure: Foliated with distinct bands (gneissic banding) of quartz, feldspar, and mafic minerals. Specific gravity: 2.6-2.9.
- Hardness
- 6-7
- Color
- Banded grey, black, pink, and white
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7. Color: Banded grey, black, pink, and white. Luster: Vitreous to dull. Structure: Foliated with distinct bands (gneissic banding) of quartz, feldspar, and mafic minerals. Specific gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary protoliths under extreme heat and pressure, often at plate boundaries or deep within mountain belts. Typically Pre-cambrian to Phanerozoic in age.
Uses & applications
Used in construction as crushed stone or dimension stone (countertops, flooring, monuments). Highly valued in the aggregate industry for durability.
Geological facts
The Acasta Gneiss in Canada is one of the oldest known crustal rock fragments on Earth, dated to approximately 4.03 billion years old.
Field identification & locations
Identify in the field by looking for distinct alternating bands of light and dark minerals without the flaky cleavage of schist. Common in mountain ranges and glacial deposits.
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