sedimentary

Concretion

Calcareous/Siliceous Concretion

Hardness: 3-7 (depending on mineralizer), Color: Tan, beige, or gray; Luster: Dull/Earthy; Crystal structure: Microcrystalline; Shape: Spherical to discoid; Cleavage: None

Hardness: 3-7 (depending on mineralizer), Color: Tan, beige, or gray
Color: Tan, beige, or gray
Luster: Dull/Earthy

Identified July 19, 2026More sedimentary →

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Physical properties

Hardness: 3-7 (depending on mineralizer), Color: Tan, beige, or gray; Luster: Dull/Earthy; Crystal structure: Microcrystalline; Shape: Spherical to discoid; Cleavage: None

Formation & geological history

Formed by the precipitation of mineral cement within the spaces between sediment particles. This often occurs around a nucleus like a leaf or shell and can date from recent times back to hundreds of millions of years ago.

Uses & applications

Primarily used as decorative display pieces, garden ornaments, or educational specimens; occasionally used in road construction when found in large quantities.

Geological facts

Concretions are often mistaken for fossils, eggs, or meteorites. Some of the most famous concretions are the Moeraki Boulders in New Zealand, which can be several meters wide.

Field identification & locations

Identify in the field by their rounded, distinct shape that differs from the surrounding bedrock matrix. Often found in shale or sandstone layers at beaches or riverbanks.