sedimentary

Siltstone

Siltstone (SiO2, Fe2O3, Al2O3 mix)

Hardness: 3-4 (Mohs); Color: Brown, drab olive, or grey; Luster: Dull/Earthy; Crystal structure: Clastic (grains smaller than sand); Cleavage: None/Planar bedding.

Hardness: 3-4 (Mohs)
Color: Brown, drab olive, or grey
Luster: Dull/Earthy

Identified June 26, 2026More sedimentary →

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Physical properties

Hardness: 3-4 (Mohs); Color: Brown, drab olive, or grey; Luster: Dull/Earthy; Crystal structure: Clastic (grains smaller than sand); Cleavage: None/Planar bedding.

Formation & geological history

Formed from the lithification of silt-sized particles deposited in low-energy environments like rivers, lakes, or floodplains. Typically Palaeozoic to Cenozoic in age.

Uses & applications

Used primarily in construction as fill material, flagstone for walkways, and occasionally as a low-grade pigment in ceramics.

Geological facts

Siltstone is often mistaken for shale, but it lacks the characteristic fissility (splitting into thin layers) that shale possesses.

Field identification & locations

Identify by its gritty feel on teeth (if tested) and lack of visible grains. Found globally in sedimentary basins. Collectors often look for examples with fossils or rhythmic bedding.