sedimentary

Siltstone

Hardness: 6-7 on Mohs scale; Color: Gray, brown, or earthy reddish-brown; Luster: Dull or earthy; Crystal structure: Clastic (microscopic grains); Cleavage: None, often breaks along bedding planes; Specific gravity: 2.6

Hardness: 6-7 on Mohs scale
Color: Gray, brown, or earthy reddish-brown
Luster: Dull or earthy

Identified June 16, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from the accumulation of silt-sized particles (larger than clay but smaller than sand) in low-energy environments like river floodplains, lakes, or deep marine basins. The silt is compacted and cemented over millions of years.

Uses & applications

Primarily used as fill material in construction, for road aggregates, and occasionally as low-quality flagstone or architectural stone when sufficiently durable.

Geological facts

Siltstone is often confused with shale, but it lacks the fine fissility (layer splitting) of shale and feels grittier when rubbed against the teeth.

Field identification & locations

Identify by its fine-grained, non-fissile texture and earthy appearance. Commonly found in sedimentary basins worldwide. Look for heavy stratification or layering in outcrops.