sedimentary

Ironstone Concretion

Siderite or Hematite Concretion

Hardness: 4-6 on Mohs scale; Color: Reddish-brown, earthy dark gray, or rusty orange; Luster: Dull, earthy, or sub-metallic; Structure: Often spherical, discoid, or irregular with concentric layering; Specific Gravity: 3.5-4.5 (dense).

Hardness: 4-6 on Mohs scale
Color: Reddish-brown, earthy dark gray, or rusty orange
Luster: Dull, earthy, or sub-metallic

Identified May 26, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the precipitation of mineral cement (typically iron oxide or carbonate) within the spaces between sediment particles around a nucleus, often before the surrounding sediment is fully lithified. Commonly found in Mesozoic or Cenozoic sandstone or shale beds.

Uses & applications

Primarily of interest to mineral collectors and geologists; historically some high-grade ironstone concretions were used as a low-grade iron ore.

Geological facts

These are often mistaken for fossils, meteorites, or even ancient artifacts due to their symmetrical shapes and the hollow 'rings' that can form when softer interiors weather away. Some 'Moqui Marbles' in Utah are a famous variety of this type of geological formation.

Field identification & locations

Identify by looking for a centered 'nucleus' or concentric rings of different colors/hardness; they are much denser than surrounding sandstone. Common in beach or desert erosion environments.

Other sedimentary specimens

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Sandstone or Siltstone

Arenite (if sandstone)

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Sandstone

Arenite (primarily SiO2)

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Shale or Slaty Mudstone

Argillaceous sedimentary rock

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

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Sandstone

Arenite

sedimentary

Sandstone (Quartz-rich)

Arenite (SiO2)

sedimentary