sedimentary

Concretion

Calcareous or ferruginous concretion

Hardness: 3-7 (variable based on cementing agent); Color: Tan, grey, brown; Luster: Dull, earthy; Structure: Spheroidal/ellipsoidal; Cleavage: None; Specific Gravity: 2.5-3.5

Hardness: 3-7 (variable based on cementing agent)
Color: Tan, grey, brown
Luster: Dull, earthy

Identified July 17, 2026More sedimentary →

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Physical properties

Hardness: 3-7 (variable based on cementing agent); Color: Tan, grey, brown; Luster: Dull, earthy; Structure: Spheroidal/ellipsoidal; Cleavage: None; Specific Gravity: 2.5-3.5

Formation & geological history

Formed by the precipitation of mineral cement in the spaces between particles of sediment, typically around a nucleus such as a fossil, leaf, or shell. They grow outwards from the center in sedimentary layers of shale or sandstone.

Uses & applications

Primarily of interest to geological collectors or used as landscaping ornaments; large specimens are sometimes showcased in museums.

Geological facts

Concretions are often mistaken for dinosaur eggs, cannonballs, or meteorites due to their unnatural-looking spherical symmetry. Some famous examples include the Moeraki Boulders in New Zealand.

Field identification & locations

Identify by its distinct round or nodules shape that differs in composition from the surrounding host rock. Commonly found in eroded creek beds or exposed sedimentary cliffs. Collectors should look for cracking or weathering on the surface that might reveal a fossil core.