sedimentary

Siltstone

Siltstone (primarily SiO2 with Al2O3)

AI-generated identification · may be incorrect

Hardness: 3-5 (Mohs scale); Color: Gray, brown, or buff; Luster: Dull/Earthy; Crystal structure: Clastic (non-visible crystals); Cleavage: None/Planar bedding; Specific gravity: 2.3-2.6

Hardness: 3-5 (Mohs scale)
Color: Gray, brown, or buff
Luster: Dull/Earthy

Identified June 22, 2026More sedimentary →

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Physical properties

Hardness: 3-5 (Mohs scale); Color: Gray, brown, or buff; Luster: Dull/Earthy; Crystal structure: Clastic (non-visible crystals); Cleavage: None/Planar bedding; Specific gravity: 2.3-2.6

Formation & geological history

Formed through the lithification of silt-sized particles deposited in low-energy environments like river floodplains, lagoons, or deep-sea basins. Most specimens date from the Paleozoic to Cenozoic eras.

Uses & applications

Used primarily as low-quality construction fill, road base, and occasionally as flagstone for landscaping or interior flooring if specifically durable.

Geological facts

Siltstone is intermediate between sandstone and shale; it has smaller grains than sandstone but lacks the fine lamination and fissility of shale.

Field identification & locations

Identify by its gritty feel on teeth (the 'grit test') despite appearing smooth. Found in sedimentary basins worldwide. Often contains small trace fossils or ripple marks.