sedimentary

River Cobble (Quartzite or Sandstone)

Clastic sedimentary rock (primarily SiO2)

Hardness: 6-7 (Mohs scale); Color: Tan, brown, or grayish-beige; Luster: Dull or earthy; Crystal structure: Microcrystalline grains; Cleavage: None; Shape: Smooth, rounded, ellipsoid due to water erosion.

Hardness: 6-7 (Mohs scale)
Color: Tan, brown, or grayish-beige
Luster: Dull or earthy

Identified July 12, 2026More sedimentary →

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

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Physical properties

Formation & geological history

Formed through the accumulation and cementation of mineral grains (likely quartz sand) over millions of years. This specific shape was created by fluvial processes, where flowing water in a river or stream tumbled the rock against others, smoothing its edges.

Uses & applications

Commonly used as decorative landscaping stones, aquarium gravel, or as a source material for construction aggregate and concrete.

Geological facts

Rocks like this act as natural 'grinders' in riverbeds, a process known as abrasion. Over thousands of years, a jagged mountain fragment can become a perfectly smooth oval cobble.

Field identification & locations

Identify by its smooth, water-worn texture and lack of sharp edges. These are found globally in active or ancient riverbeds, beaches, and glacial deposits. Collectors often use them for rock painting or as 'worry stones'.

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Sandstone or Siltstone

Arenite (if sandstone)

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Brown Sandstone (River Rock)

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Mudstone

Argillite / Siliciclastic sedimentary rock

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Cone-in-cone structure (Limestone)

Cone-in-cone structure (Secondary Sedimentary Structure)

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Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

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Sandstone

Arenite (primarily SiO2)

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