sedimentary

Concretion

Siderite or Ironstone Concretion

Hardness 3.5 - 5.5 depending on iron content; Color: rusty brown, tan, or dark grey; Luster: earthy to dull; Crystal structure: massive or cryptocrystalline; Cleavage: none; Specific gravity: 3.0 - 4.0; typically rounded or nodular shape.

Color: rusty brown, tan, or dark grey
Luster: earthy to dull

Identified June 27, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the precipitation of mineral cement in the spaces between particles in sediment, often around a nucleus like a leaf or bone. They form early in the burial history of sediment before it turns into hard rock.

Uses & applications

Used primarily for geological study, landscaping, and as curios for collectors. Some larger ones are used in construction or as low-grade iron ore.

Geological facts

People often mistake small, round concretions for dinosaur eggs or cannonballs. Some 'septarian' concretions feature beautiful internal cracks filled with calcite crystals.

Field identification & locations

Look for rounded, heavy, or oddly shaped nodules sticking out of softer sedimentary layers like shale or sandstone. Often found along riverbanks and in eroded canyons. They are distinctly harder than the surrounding 'host' rock.