metamorphic
Gneiss (Metamorphic Rock)
Gneiss (variable mineral formula primarily containing Quartz, Feldspar, and Mica)
Hardness: 6-7 on Mohs scale. Color: Gray, pink, white, or black. Luster: Dull to sub-vitreous. Structure: Coarse-grained, foliated with light/dark banding (gneissic banding). Cleavage: Poor; fractures along grain boundaries. Specific Gravity: 2.6 - 2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, pink, white, or black
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Gray, pink, white, or black. Luster: Dull to sub-vitreous. Structure: Coarse-grained, foliated with light/dark banding (gneissic banding). Cleavage: Poor; fractures along grain boundaries. Specific Gravity: 2.6 - 2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous (granite) or sedimentary (shale/mudstone) rocks. This process involves intense heat and pressure deep within the Earth's crust, often during mountain-building events (orogenies). Ages vary from millions to billions of years old.
Uses & applications
Used primarily in the construction industry as crushed stone for road bases. High-quality decorative specimens are used as building facing, kitchen countertops (often sold as 'granite'), and as gravitational ballast for landscaping.
Geological facts
Gneiss is some of the oldest rock on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The term comes from the Middle High German verb 'gneist' meaning 'to spark', because the rock can glisten under light.
Field identification & locations
Identify in the field by looking for distinct light and dark parallel bands (segregated minerals). It will not fizz with acid (unlike marble) and is much harder than shale or slate. Common in mountain ranges like the Appalachians, Alps, and Himalayas.
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