igneous or sedimentary
River Stone (Basalt or Siltstone with banding)
Polished Basaltic or Siliciclastic River Cobble
Hardness: 5-7 (Mohs scale), Color: Dark grey to charcoal with lighter horizontal banding, Luster: Dull to waxy when polished, Crystal structure: Aphanitic (fine-grained), Specific gravity: 2.6-3.0
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Physical properties
Hardness: 5-7 (Mohs scale), Color: Dark grey to charcoal with lighter horizontal banding, Luster: Dull to waxy when polished, Crystal structure: Aphanitic (fine-grained), Specific gravity: 2.6-3.0
Formation & geological history
Formed through either volcanic activity (if basaltic) or sediment accumulation (if siltstone). The rounded, smooth shape is a result of fluvial erosion (water transport in rivers or streams) over thousands of years.
Uses & applications
Used primarily for decorative landscaping, Zen gardens, massage stones (hot stone therapy), and as aquarium substrate.
Geological facts
River stones have been used for centuries in various cultures to create 'cairns' or trail markers. Their smooth texture is caused by 'saltation,' where rocks bounce along river bottoms, chipping away sharp edges.
Field identification & locations
Identify by its smooth, elliptical shape and lack of sharp edges. To distinguish between basalt and siltstone in the field, look for fine layering (sedimentary) versus a dense, uniform matrix (igneous).
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