sedimentary

Siltstone

Siltstone (primarily SiO2, Al2O3)

Hardness: 3-5 on Mohs scale; Color: Grayish-brown; Luster: Dull/Earthy; Structure: Fine-grained clastic; Cleavage: Non-existent, though it may split along bedding planes.

Hardness: 3-5 on Mohs scale
Color: Grayish-brown
Luster: Dull/Earthy

Identified July 20, 2026More sedimentary →

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Physical properties

Hardness: 3-5 on Mohs scale; Color: Grayish-brown; Luster: Dull/Earthy; Structure: Fine-grained clastic; Cleavage: Non-existent, though it may split along bedding planes.

Formation & geological history

Formed by the accumulation and lithification of silt-sized particles (between sand and clay size) in low-energy environments like river floodplains, lagoons, or deep-sea basins.

Uses & applications

Primarily used as a low-quality fill material in construction, for flagstones, or as a decorative stone in landscaping. Some varieties are used as whetstones.

Geological facts

Siltstone is often mistaken for shale, but it lacks the 'fissility' (tendency to split into thin sheets) that defines true shale. It feels grittier when rubbed against teeth compared to clay-rich shale.

Field identification & locations

Identified by its fine-grained texture that is too small to see individual grains with the naked eye, yet feels slightly gritty. Common in sedimentary basins worldwide.