sedimentary

Siltstone

Siltstone (primarily composed of microcrystalline Quartz and Feldspar)

Hardness: 3-4 on Mohs scale; Color: Gray, brown, or tan with visible bedding; Luster: Dull/Earthy; Crystal structure: Non-crystalline visible grain; Cleavage: None (exhibits conchoidal or irregular fracture); Specific gravity: 2.3–2.6.

Hardness: 3-4 on Mohs scale
Color: Gray, brown, or tan with visible bedding
Luster: Dull/Earthy

Identified March 18, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from the accumulation and lithification of silt-sized particles (0.0039 to 0.0625 mm) in low-energy environments such as river floodplains, lakes, or deep marine basins. Common across various geological eras from the Paleozoic to the Cenozoic.

Uses & applications

Used primarily as low-grade construction aggregate, flagstone for landscaping, and occasionally as a base material for roads or as a filler in industrial products.

Geological facts

Siltstone is intermediate between sandstone and shale; it lacks the fissility (layer splitting) of shale but has much finer grains than sandstone. It often preserves delicate trace fossils like worm burrows.

Field identification & locations

Identify in the field by its gritty feel (less smooth than shale but smoother than sandstone) and the lack of visible individual grains to the naked eye. Found globally in sedimentary basins. Collectors often look for examples with distinct laminations or fossils.

Other sedimentary specimens

See all

Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary

Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary

Sandstone (Quartz-rich)

Arenite (SiO2)

sedimentary

Sandstone (with man-made markings)

Arenite (composed primarily of Quartz, SiO2)

sedimentary