sedimentary

River Cobble (Quartzite or Weathered Sandstone)

Arenite / Metasandstone (SiO2)

AI-generated identification · may be incorrect

Hardness: 6.5-7 on Mohs scale; Color: Tan, beige, and dark grey mottling; Luster: Dull to sub-vitreous when wet; Structure: Granular or microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Hardness: 6
Color: Tan, beige, and dark grey mottling
Luster: Dull to sub-vitreous when wet

Identified July 4, 2026More sedimentary →

Explore River Cobble (Quartzite or Weathered Sandstone) in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed from the lithification of sand deposits, likely subjected to river transport which rounded its edges into a 'cobble' shape. May have experienced low-grade metamorphism (quartzite) depending on its depth of burial and heat exposure.

Uses & applications

Used primarily as decorative landscaping stone, aggregate for construction, or as river rock for drainage and erosion control. Occasionally collected for lapidary tumbling.

Geological facts

The rounded shape indicates a long history of hydraulic transport, where the rock was tumbled against others in a stream bed, smoothing its sharp edges. The pitting on the surface suggests the erosion of softer mineral grains or dissolution over time.

Field identification & locations

Identify in the field by its rounded, water-worn shape and high hardness (cannot be scratched by a steel knife). Commonly found in current or ancient riverbeds, alluvial fans, and coastal regions.