sedimentary
River Cobble (Sandstone or Greywacke)
Arenite / Greywacke
Hardness: 6-7 (Mohs); Color: Grey to brownish-grey; Luster: Dull/Earthy; Texture: Fine to medium-grained; Structure: Massive, well-rounded by water erosion.
- Hardness
- 6-7 (Mohs)
- Color
- Grey to brownish-grey
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 (Mohs); Color: Grey to brownish-grey; Luster: Dull/Earthy; Texture: Fine to medium-grained; Structure: Massive, well-rounded by water erosion.
Formation & geological history
Formed via the accumulation and cementation of sand-sized mineral particles or rock fragments. The rounded shape indicates significant transport in a high-energy fluvial environment (river) during the Holocene or Pleistocene.
Uses & applications
Commonly used as construction aggregate, decorative landscaping stone, or as a tool in ancient civilizations (hammerstone).
Geological facts
The regional coordinates (near Toulouse, France) are consistent with the Garonne river basin, where large deposits of alluvial pebbles and cobbles from the Pyrenees are frequently found.
Field identification & locations
Identified by its sandy texture and smooth, rounded edges. Look for these in riverbeds or prehistoric terrace deposits. They are excellent for testing mineral hardness.
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