sedimentary

Siltstone

Siltstone (primarily SiO2 with Al2O3)

Hardness: 3-4 Mohs; Color: Grey-brown to olive; Luster: Dull/Earthy; Structure: Clastic, fine-grained; Cleavage: None, can show fissility if transitioning to shale.

Hardness: 3-4 Mohs
Color: Grey-brown to olive
Luster: Dull/Earthy

Identified July 6, 2026More sedimentary →

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Physical properties

Hardness: 3-4 Mohs; Color: Grey-brown to olive; Luster: Dull/Earthy; Structure: Clastic, fine-grained; Cleavage: None, can show fissility if transitioning to shale.

Formation & geological history

Formed by the deposition and lithification of silt-sized particles (larger than clay but smaller than sand) in low-energy aquatic environments like river floodplains or lake beds.

Uses & applications

Commonly used as fill material in construction or as a minor decorative stone; some varieties are used for sharpening tools or as low-quality flagstone.

Geological facts

Siltstone is intermediate between sandstone and shale. While it looks like shale, it lacks the distinct layered splitting (fissility) and feels slightly grittier to the touch.

Field identification & locations

Identify in the field by its fine-grained texture that feels slightly gritty on the teeth (a common geologist field test) and its lack of visible grains to the naked eye. Commonly found in sedimentary basins worldwide.