metamorphic
Gneiss (Cobble)
Gneiss
Hardness: 6-7 on Mohs scale; Color: typically banded gray, white, and black; Luster: vitreous to dull; Crystal structure: granular/foliated; Cleavage: none (fractures); Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- typically banded gray, white, and black
- Luster
- vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: typically banded gray, white, and black; Luster: vitreous to dull; Crystal structure: granular/foliated; Cleavage: none (fractures); Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous (like granite) or sedimentary (like shale) rocks under intense heat and pressure. This specimen shows smoothing from water transport, likely in a river or glacial environment.
Uses & applications
Used as crushed stone for road construction, decorative garden landscaping (river rock), and occasionally as dimension stone for counter tops if the banding is aesthetic.
Geological facts
Gneiss is one of the oldest rock types on Earth, with some specimens like the Acasta Gneiss dating back over 4 billion years. The banding is caused by the segregation of light and dark minerals during intense pressure.
Field identification & locations
Identify by looking for 'gneissic banding' (layers of different colors) and a salt-and-pepper appearance. Commonly found in metamorphic terrains like the Appalachian or Rocky Mountains, often in riverbeds as rounded cobbles.
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