metamorphic
Gneiss (River Cobble)
Metamorphic Gneiss
Hardness: 6-7 on Mohs scale; Color: Gray, tan, and dark bands; Luster: Dull to slightly vitreous; Crystal structure: Coarse-grained foliated; Cleavage: Poor; Specific gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, tan, and dark bands
- Luster
- Dull to slightly vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, tan, and dark bands; Luster: Dull to slightly vitreous; Crystal structure: Coarse-grained foliated; Cleavage: Poor; Specific gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme heat and pressure. This specific specimen has been rounded and smoothed by water transport in a river or glacial environment.
Uses & applications
Commonly used as crushed stone for road construction, decorative landscaping (river rocks), and occasionally as dimension stone for architectural facades.
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 distinct banding is called 'gneissic banding'.
Field identification & locations
Identify in the field by looking for 'salty and pepper' banding or stripes that are distinct from the layers in sedimentary rock. They are commonly found in glacial till or riverbeds in mountainous regions.
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