metamorphic
Gneiss
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Gray, pink, white, and black banded; Luster: Dull to vitreous; Structure: Foliated (gneissic banding); Cleavage: Poor; Specific Gravity: 2.6-3.0
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, pink, white, and black banded
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, pink, white, and black banded; Luster: Dull to vitreous; Structure: Foliated (gneissic banding); Cleavage: Poor; Specific Gravity: 2.6-3.0
Formation & geological history
Formed by high-grade regional metamorphism of igneous or sedimentary protoliths under intense heat and pressure, often at deep crustal levels during orogenic (mountain-building) events.
Uses & applications
Primarily used as a durable construction material, crushed stone for road bases, architectural facing stone, and decorative landscaping rock.
Geological facts
Gneiss is among the oldest known rocks on Earth, with some specimens like the Acasta Gneiss dating back over 4 billion years. The banding is caused by the segregation of light minerals (quartz/feldspar) and dark minerals (mica/amphibole).
Field identification & locations
Identify by distinct light and dark compositional banding (foliation) that is thicker than schist. It is commonly found in the cores of mountain ranges and stable continental shields. Collectors look for well-defined bands or garnet inclusions.
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