sedimentary

Siltstone

Siltstone (primarily SiO2 with Al2O3)

AI-generated identification · may be incorrect

Hardness: 6-7 (Mohs); Color: Grey, brown, or reddish-brown; Luster: Dull/Earthy; Crystal structure: Clastic (non-visible crystals); Cleavage: None; Specific gravity: 2.6

Hardness: 6-7 (Mohs)
Color: Grey, brown, or reddish-brown
Luster: Dull/Earthy

Identified July 1, 2026More sedimentary →

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Physical properties

Hardness: 6-7 (Mohs); Color: Grey, brown, or reddish-brown; Luster: Dull/Earthy; Crystal structure: Clastic (non-visible crystals); Cleavage: None; Specific gravity: 2.6

Formation & geological history

Formed through the accumulation and lithification of silt-sized particles in low-energy aquatic environments like river floodplains or lake beds. Age varies widely from Precambrian to Cenozoic.

Uses & applications

Commonly used as a construction aggregate, for road base, filler material, and occasionally for interior decorative stone or floor tiles.

Geological facts

Siltstone is intermediate between sandstone and shale. Unlike shale, it lacks the 'fissility' or the ability to split into thin layers, breaking instead into irregular blocks.

Field identification & locations

Identify in the field by its gritty feel on the teeth (if sampled) but lack of visible grains to the naked eye. Found in sedimentary basins worldwide.