metamorphic
Gneiss (Metamorphic Cobble)
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Gray, white, and black banded; Luster: Dull to sub-vitreous; Crystal structure: Coarse-grained crystalline; Cleavage: None (bears foliation instead); Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, white, and black banded
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, white, and black banded; Luster: Dull to sub-vitreous; Crystal structure: Coarse-grained crystalline; Cleavage: None (bears foliation instead); Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through the high-pressure, high-temperature regional metamorphism of pre-existing igneous (granite) or sedimentary rocks. This specific specimen is a water-worn cobble, likely shaped by river or glacial transport over thousands of years.
Uses & applications
Used primarily in construction as crushed stone for road bases, concrete aggregate, and occasionally as dimension stone for flooring or countertops. Not common in jewelry but valued by collectors for teaching geological layering.
Geological facts
Gneiss contains the oldest known rocks on Earth, such as the Acasta Gneiss in Canada, which is approximately 4 billion years old. The term 'gneiss' comes from a Middle High German verb meaning 'to spark' or 'to glisten.'
Field identification & locations
Identify in the field by its distinct 'gneissic banding' (alternating light and dark layers) and coarse grain size. Often found in riverbeds, glacial tills, and mountain ranges. It will not fizz with acid, distinguishing it from marble.
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