metamorphic or sedimentary

River Cobble (likely Quartzite or Limestone)

SiO2 (Quartzite dominant) or CaCO3 (Limestone dominant)

AI-generated identification · may be incorrect

Hardness: 7 (if quartzite) or 3 (if limestone); Color: Grey/Tan/White; Luster: Dull to Waxy; Structure: Finely crystalline to microcrystalline; Surface: Smooth, rounded, water-worn.

Hardness: 7 (if quartzite) or 3 (if limestone)
Color: Grey/Tan/White
Luster: Dull to Waxy

Identified April 6, 2026More metamorphic or sedimentary →

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Physical properties

Hardness: 7 (if quartzite) or 3 (if limestone); Color: Grey/Tan/White; Luster: Dull to Waxy; Structure: Finely crystalline to microcrystalline; Surface: Smooth, rounded, water-worn.

Formation & geological history

Formed through the smoothing of rock fragments by water transport (abrasion) in rivers or ocean shorelines. Geologically, these can range from millions to billions of years old before being weathered.

Uses & applications

Landscaping, decorative garden stone, construction aggregate, and sometimes used in stone tumbling for hobbyists.

Geological facts

River stones get their smooth shape from 'attrition,' a process where rocks collide into each other in a current, wearing down jagged edges over time.

Field identification & locations

Identify by its smooth, rounded 'egg' shape and lack of sharp edges. Found in riverbeds, glacial tills, and beaches. A simple acid test (vinegar) can determine if it is limestone (it will fizz) or quartzite (no reaction).