igneous or sedimentary
River Rock (Basalt or Sandstone)
Clastic Sedimentary Rock or Extrusive Mafic Igneous Rock
Hardness: 5.5-7 Mohs; Color: Dark grey, brown, or weathered tan; Luster: Dull/Earthy; Texture: Fine-grained to slightly granular; Cleavage: None (subconchoidal fracture).
- Hardness
- 5
- Color
- Dark grey, brown, or weathered tan
- Luster
- Dull/Earthy
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Physical properties
Hardness: 5.5-7 Mohs; Color: Dark grey, brown, or weathered tan; Luster: Dull/Earthy; Texture: Fine-grained to slightly granular; Cleavage: None (subconchoidal fracture).
Formation & geological history
Formed either through volcanic activity (basalt) or the lithification of sand/silt (sandstone), later shaped into a rounded 'cobble' by terminal water erosion in a river or coastal environment.
Uses & applications
Commonly used in landscaping, construction aggregate, aquarium decor, or as decorative garden stones.
Geological facts
Rocks with natural holes, often called 'Hag Stones' or 'Odin Stones', are traditionally believed in folklore to ward off evil or provide 'second sight' when looking through the hole.
Field identification & locations
Identify by its rounded, water-worn edges and smooth surface. Look for holes formed by 'potholing' (abrasion by smaller pebbles in water) or the weathering out of softer mineral inclusions. Found worldwide in riverbeds and beaches.
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