sedimentary

Graywacke Pebble (River Stone)

Graywacke (Lithic wacke)

Hardness: 6-7 on Mohs scale. Color: Medium to dark gray. Luster: Dull to earthy. Crystal structure: Clastic (sand-sized grains in a finer matrix). Cleavage: None. Specific gravity: 2.6-2.7. Smooth, water-worn texture.

Hardness: 6-7 on Mohs scale
Color: Medium to dark gray
Luster: Dull to earthy

Identified May 25, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from the deposition of poorly sorted sediment in deep-marine environments through turbidity currents. They often date back to the Paleozoic or Mesozoic eras. This specific specimen was shaped into a smooth pebble through fluvial (river) or coastal erosion processes.

Uses & applications

Used primarily as construction aggregate, road base, and in decorative landscaping or as garden river stones. Polished versions are sometimes used for massage stones or decorative accents.

Geological facts

Graywacke is often called 'dirty sandstone' because of its mixture of clay, silt, and poorly sorted sand grains. It is a common component of accretionary wedges in subduction zones.

Field identification & locations

Identify in the field by its hardness (won't be scratched by a steel nail), gray matte appearance, and absence of visible layering compared to siltstone or shale. Commonly found in riverbeds and mountainous coastlines worldwide.

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