sedimentary
River Rock (Greywacke/Siltstone)
Siltstone / Argillaceous Greywacke
Hardness: 6-7 (Mohs). Color: Dark grey, charcoal, or olive-drab. Luster: Dull/Earthy. Structure: Massive or subtly bedded. Specific Gravity: 2.6-2.7. Very fine-grained texture with a smooth, water-worn surface.
- Hardness
- 6-7 (Mohs)
- Color
- Dark grey, charcoal, or olive-drab
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 (Mohs). Color: Dark grey, charcoal, or olive-drab. Luster: Dull/Earthy. Structure: Massive or subtly bedded. Specific Gravity: 2.6-2.7. Very fine-grained texture with a smooth, water-worn surface.
Formation & geological history
Formed through the lithification of mud and silt deposits in low-energy environments like river deltas or deep-sea fans. This specific specimen has been shaped and polished over time by fluvial (river) or glacial abrasion.
Uses & applications
Commonly used in landscaping, construction aggregate, aquarium decor, and as 'worry stones' or for rock painting.
Geological facts
River rocks like this are often hundreds of millions of years old. Their rounded shape is a physical record of the distance they have traveled downstream; the smoother the stone, the longer the journey.
Field identification & locations
Identify in the field by its smooth, ellipsoidal shape and lack of visible crystals. It is dense and will not scratch easily with a steel knife. Found in riverbeds, beaches, and glacial till worldwide.
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