sedimentary

Siltstone

Siltstone (composed of mixed silicate minerals and clay)

Hardness: 3-4 (Mohs scale); Color: Dark grey, charcoal, or brown; Luster: Dull/Earthy; Texture: Fine-grained, gritty feel (finer than sandstone but coarser than shale); Cleavage: None, often breaks along bedding planes.

Hardness: 3-4 (Mohs scale)
Color: Dark grey, charcoal, or brown
Luster: Dull/Earthy

Identified July 12, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from the accumulation and lithification of silt-sized particles (0.0039 to 0.063 mm). Usually deposited in low-energy environments like river floodplains, lakes, or deep marine areas. Age varies by location.

Uses & applications

Used primarily as low-grade construction fill, road base, or flagstone. Historically used for sharpening tools if the grit was appropriate. Very little industrial value compared to other rocks.

Geological facts

Siltstone is often overlooked because it mimics shale, but unlike shale, it lacks the 'fissility' or the ability to split into thin, perfect sheets. It is a major component of the sedimentary record worldwide.

Field identification & locations

Identify by its gritty feel against the teeth (the 'crunch' test used by geologists) and its lack of visible grains to the naked eye. Commonly found in riverbeds and sedimentary basins. In this specific shape, it may be a naturally weathered fragment or a lithic flake.

Other sedimentary specimens

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Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Brown Sandstone (River Rock)

Arenite

sedimentary

Mudstone

Argillite / Siliciclastic sedimentary rock

sedimentary

Cone-in-cone structure (Limestone)

Cone-in-cone structure (Secondary Sedimentary Structure)

sedimentary

Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary