sedimentary

Concretion

Sedimentary Concretion (Ironstone / Calcite Concretion)

Hardness: 4 to 7 (depending on cementing minerals such as calcite or quartz), Color: Brown, reddish-brown, or yellowish-brown, Luster: Dull to earthy, Crystal structure: Microcrystalline or amorphous matrix, Cleavage: None, Specific Gravity: 2.5 - 3.0

Identified July 28, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in sedimentary rock layers (often sandstone, shale, or limestone) through the precipitation of mineral cement within the spaces between sediment grains, typically around a nucleus such as a shell or plant fragment.

Uses & applications

Primarily collected as geological curiosities, specimens for educational purposes, or used in lapidary work when cracked open to reveal crystalline interiors.

Geological facts

Concretions are frequently mistaken for fossils, dinosaur eggs, or man-made artifacts due to their unusual, often symmetrical and rounded shapes. They grow outward from a central point in layers.

Field identification & locations

Found weathering out of sedimentary cliffs and riverbeds worldwide. Identified by their distinct nodular, spherical, or bell-like shapes and differential weathering compared to the host rock.

Other sedimentary specimens

See all

Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Sedimentary Sandstone

Arenite (Sedimentary Rock)

sedimentary

Fossiliferous Concretion

Septarian Concretion containing Brachiopods/Crinoids

fossil

Banded Siltstone / Mudstone Pebble

Siltstone / Argillite

sedimentary

Shale (or Mudstone/Siltstone)

Sedimentary Mudrock (SiO2, Al2O3 base with clay minerals)

sedimentary

Shale

Shale (clastic sedimentary rock)

sedimentary