sedimentary

River Pebble (Greywacke or Chert mix)

Silicon Dioxide (SiO2) with varying mineral impurities

Hardness: 6.5-7 (Mohs scale), Color: Dark grey, charcoal, and mottled brown-white, Luster: Dull to slightly waxy when polished, Crystal Structure: Microcrystalline or Cryptocrystalline, Cleavage: None (conchoidal fracture)

Hardness: 6

Identified June 12, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from the lithification of sand, silt, and clay in deep-water marine environments or as chemical precipitates. This specific specimen has been significantly rounded and smoothed by fluvial or alluvial processes (water erosion in a stream or river).

Uses & applications

Mainly used in construction as aggregate, landscaping, decorative garden stones, and occasionally in lapidary for tumbling to create polished stones.

Geological facts

River pebbles act as geological time capsules, often containing fragments of much older rocks from further upstream. Their smooth, rounded shape is a direct result of 'abrasion,' where stones collide and wear down while being transported by water over miles.

Field identification & locations

Identify by its water-worn, rounded shape and smooth texture. Common in river beds, beaches, and glacial deposits. For identification, check for lack of visible large crystals and high hardness (will scratch glass).

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