sedimentary

Concretion

Septarian or Siderite Concretion

Hardness: 3-5 (Mohs scale), Color: tan, grey, or brownish-red. Luster: dull to earthy. Structure: often spherical or discoid with concentric layers. Cleavage: none (fractures irregularly).

Hardness: 3-5 (Mohs scale), Color: tan, grey, or brownish-red
Color: tan, grey, or brownish-red
Luster: dull to earthy

Identified July 21, 2026More sedimentary →

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Physical properties

Hardness: 3-5 (Mohs scale), Color: tan, grey, or brownish-red. Luster: dull to earthy. Structure: often spherical or discoid with concentric layers. Cleavage: none (fractures irregularly).

Formation & geological history

Formed by the precipitation of mineral cement within the spaces between particles in a sediment, often around a nucleus like a shell or leaf. They form in marine or terrestrial sedimentary basins over millions of years.

Uses & applications

Mainly of interest to collectors and geologists. Large decorative ones are used in landscaping or sliced and polished for display as 'septarian nodules' if they contain internal crystals.

Geological facts

Concretions are often mistaken for dinosaur eggs or fossils due to their organic shapes. The Moeraki Boulders in New Zealand are world-famous examples of massive concretions.

Field identification & locations

Look for rounded, hard masses that differ in texture and hardness from the surrounding soil or soft rock. They are commonly found in beds of shale, siltstone, or sandstone along riverbanks and coastlines.