metamorphic
Gneiss
Foliated Metamorphic Rock (Gneiss)
Hardness: 6-7. Color: Banded grey, white, pink, and dark brown/black. Luster: Dull to vitreous. Crystal structure: Coarse-grained phaneritic. Cleavage: Poor, but exhibits distinct foliation. Specific gravity: 2.6-2.9.
- Hardness
- 6-7
- Color
- Banded grey, white, pink, and dark brown/black
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7. Color: Banded grey, white, pink, and dark brown/black. Luster: Dull to vitreous. Crystal structure: Coarse-grained phaneritic. Cleavage: Poor, but exhibits distinct foliation. Specific gravity: 2.6-2.9.
Formation & geological history
Formed by high-grade regional metamorphism of igneous or sedimentary protoliths. Subjected to intense heat and pressure, typically found in the roots of mountain ranges or Precambrian shields. Ages range from millions to billions of years (e.g., Acasta Gneiss is 4 billion years old).
Uses & applications
Used extensively as a crushed stone in road construction and as aggregate. Higher quality, colorful specimens are cut and polished for architectural stone, countertops, and decorative landscaping.
Geological facts
Gneiss is one of the oldest rock types on Earth. The 'Acasta Gneiss' in Canada is approximately 4.03 billion years old, representing some of the earliest crustal material identified.
Field identification & locations
Identify in the field by looking for 'gneissose banding'—alternating layers of light-colored minerals (quartz/feldspar) and dark-colored minerals (biotite/hornblende). Unlike schist, it does not split easily along planes. Often found in glaciated riverbeds or mountain outcrops.
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