sedimentary
Riverstone (likely Graywacke or Siltstone)
Sedimentary sandstone (Graywacke), SiO2 dominant
Hardness: 6-7 (Mohs scale), Color: Dull gray-green to olive, Luster: Sub-vitreous to earthy, Structure: Fine-grained clastic, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Dull gray-green to olive, Luster: Sub-vitreous to earthy, Structure: Fine-grained clastic, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
Formation & geological history
Formed from the lithification of clay, silt, and sand deposits in marine or river environments. This specific specimen has been further smoothed and rounded by river erosion over thousands of years.
Uses & applications
Decorative landscaping, aquarium accents, construction aggregate, and occasionally used as a natural canvas for rock painting.
Geological facts
Riverstones are iconic for their smooth texture, which is a result of fluvial abrasion, where stones collide and rub against each other in moving water. Graywacke belongs to the 'dirty' sandstone category because it contains more than 15% matrix.
Field identification & locations
Identify by its rounded, oblong shape and dull surface that becomes vibrant when wet. Found in riverbeds, glacial moraines, and beaches worldwide. To test, check its hardness against a steel file to differentiate from softer shale.
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