sedimentary

Ironstone Concretion / Chert Nodules

Iron-rich Sedimentary Concretion (Fe2O3 / SiO2 mix)

Hardness: 6-7 on Mohs scale. Color: Yellowish-brown, tan, dark brown, rust-colored. Luster: Earthy, dull, or slightly waxy. Structure: Microcrystalline, nodular, rounded to irregular shapes. Cleavage: None (conchoidal or irregular fracture).

Hardness: 6-7 on Mohs scale
Color: Yellowish-brown, tan, dark brown, rust-colored
Luster: Earthy, dull, or slightly waxy

Identified July 27, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by precipitation of iron minerals (like goethite or limonite) and silica around a nucleus within sedimentary rock beds such as sandstone, shale, or limestone during diagenesis.

Uses & applications

Used primarily as decorative rocks, educational specimens for rockhounding enthusiasts, or low-grade iron ore historical study.

Geological facts

Concretions can often look like fossilized eggs or bones, leading to misidentification by amateur collectors. They often feature concentric layers inside when broken open.

Field identification & locations

Identify in the field by their rounded, weathered, yellowish-tan to rust-brown outer rind and heavy, dense feel compared to surrounding sedimentary rock. Common in gravel bars, stream beds, and sedimentary outcrops worldwide.

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Sandstone or Siltstone

Arenite (if sandstone)

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Sedimentary Sandstone

Arenite (Sedimentary Rock)

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Fossiliferous Concretion

Septarian Concretion containing Brachiopods/Crinoids

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Banded Siltstone / Mudstone Pebble

Siltstone / Argillite

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Shale

Shale (clastic sedimentary rock)

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Shale

Sedimentary Shale

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