sedimentary

Siltstone

Siltstone (primarily SiO2 with Al2O3)

Hardness: 3-5 (Mohs scale), Color: Gray, tan, and reddish-brown, Luster: Dull to earthy, Structure: Fine-grained clastic, Cleavage: None (conchoidal or irregular fracture), Specific Gravity: 2.3-2.6

Identified June 27, 2026More sedimentary →

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Physical properties

Hardness: 3-5 (Mohs scale), Color: Gray, tan, and reddish-brown, Luster: Dull to earthy, Structure: Fine-grained clastic, Cleavage: None (conchoidal or irregular fracture), Specific Gravity: 2.3-2.6

Formation & geological history

Formed from the accumulation and lithification of silt-sized particles (larger than clay but smaller than sand) in low-energy environments like river floodplains, deltas, or lakes. Common throughout various geological eras.

Uses & applications

Primarily used in construction as fill material, road base, and occasionally as interior flooring or garden stones if durable enough.

Geological facts

Siltstone is often mistaken for shale, but it lacks the characteristic fissility (ability to split into thin layers) that shale possesses. It represents a transition between sandstone and shale.

Field identification & locations

Identify in the field by its fine-grained texture that feels slightly gritty on the teeth (a common field test) but smoother than sandstone. Commonly found in sedimentary basins worldwide.