metamorphic
River Cobble (likely Metasedimentary)
Metasedimentary Rock / Quartz-rich Sandstone
Hardness: 6-7 on Mohs scale. Color: Dark grey to brown with moss/algae growth. Luster: Dull to earthy when dry, glossy when wet. Crystal structure: Fine-grained to micro-crystalline. Specific gravity: 2.6-2.7.
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey to brown with moss/algae growth
- Luster
- Dull to earthy when dry, glossy when wet
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Dark grey to brown with moss/algae growth. Luster: Dull to earthy when dry, glossy when wet. Crystal structure: Fine-grained to micro-crystalline. Specific gravity: 2.6-2.7.
Formation & geological history
Formed from the lithification of ancient river or marine sediments (sand and silt) followed by low-grade metamorphism (heat/pressure) over millions of years. This specific specimen has been further shaped and rounded by river erosion.
Uses & applications
Primarily used in landscaping, decorative river rock, and construction aggregate. Larger specimens are used for natural garden paths or rustic masonry.
Geological facts
The moss and algae growing on the side indicate it has spent significant time in a stable, moist environment. River rocks are often much older than the current riverbed, having been transported from mountainous outcrops far upstream.
Field identification & locations
Identify in the field by its rounded shape (indicative of water transport) and high hardness (scratching glass or steel). Commonly found in current and ancient riverbeds, deltas, and glacial till.
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