sedimentary

Siltstone Pebble

Siltstone (composed of siliciclastic grains)

Hardness: 3.5–5 on Mohs scale; Color: Gray to light grayish-green; Luster: Dull/Earthy; Structure: Clastic, fine-grained; Cleavage: None (conchoidal to irregular fracture); Specific Gravity: 2.1–2.6

Hardness: 3
Color: Gray to light grayish-green
Luster: Dull/Earthy

Identified July 21, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from the lithification of silt-sized particles (larger than clay but smaller than sand). These particles typically settle in slow-moving water environments like floodplains, lagoons, or deep river basins. This specimen has been rounded by fluvial (water) erosion.

Uses & applications

Primarily used as construction aggregate, fill material, and sometimes for landscaping stone. It lacks the durability for fine jewelry but is a common educational sample for geology.

Geological facts

Siltstone is often mistaken for shale, but it lacks the characteristic fissility (ability to split into thin sheets) of shale. It is actually a very common bridge between sandstone and mudstone in the rock cycle.

Field identification & locations

Identify in the field by its gritty feel on the teeth (unlike clay) and its extremely fine-grained, uniform appearance compared to sandstone. Commonly found in riverbeds and coastal areas worldwide.

Other sedimentary specimens

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Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Sedimentary Sandstone

Arenite (Sedimentary Rock)

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Fossiliferous Concretion

Septarian Concretion containing Brachiopods/Crinoids

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Shale

Shale (clastic sedimentary rock)

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Shale

Argillaceous sedimentary rock

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Mudstone

Argillite / Siliciclastic sedimentary rock

sedimentary