River Cobble (Quartzite-rich Sandstone)

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Brown, tan, and grey with reddish iron staining; Luster: Dull/Earthy; Crystal Structure: Granular/Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Formation & Geological History

Formed from the lithification of sand deposits, likely during the Paleozoic or Mesozoic eras. This specific specimen has been significantly weathered and smoothed by transport in a river or glacial environment, a process known as 'rounding'.

Uses & Applications

Common river stones are used primarily in landscaping, decorative stone masonry, and construction aggregate. Extremely hard variants like quartzite are used as railway ballast.

Geological Facts

The reddish-orange hues on the surface are 'desert varnish' or iron oxide staining, caused by long-term exposure to water and oxygen. The smooth, tear-drop shape suggests it traveled many miles in a high-energy water environment.

Field Identification & Locations

Identify in the field by its smooth, water-worn texture and 'pitting' from collisions in the stream bed. Commonly found in riverbeds, glacial tills, and alluvial fans. Collectors often look for these as 'base stones' for painting or as garden accents.

Identified on: 4/22/2026

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