metamorphic
River Pebble (Likely Quartzite or Chalcedony)
Silicon Dioxide (SiO2) - granular metamorphic variety
Hardness: 7 (Mohs scale), Color: tan to yellowish-brown, Luster: dull to waxy (smooth from tumbling), Crystal Structure: Macrocrystalline to microcrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: ~2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: tan to yellowish-brown, Luster: dull to waxy (smooth from tumbling), Crystal Structure: Macrocrystalline to microcrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: ~2.65
Formation & geological history
Formed through the metamorphism of sandstone (quartzite) or silica precipitation (chalcedony). The rounded shape is the result of mechanical weathering and transport by moving water over hundreds to thousands of years.
Uses & applications
Used in landscaping, decorative ground cover, aquarium gravel, and occasionally as a source of high-purity silica for construction materials or industrial filters.
Geological facts
River pebbles reach their smooth, rounded shape via a process called abrasion. This specific specimen’s yellowish tint may be caused by inclusions of iron oxides (limonite or goethite) during its formation or surface staining.
Field identification & locations
Identify in the field by its rounded, water-worn edges, hardness (it will scratch glass), and lack of visible grains. Found globally in riverbanks, coastal beaches, and ancient glacial deposits.
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