sedimentary

Concretion

Septarian concretion (likely containing Calcite or Siderite)

Hardness: 3-4 (Mohs); Color: Tan, brown, or gray exterior with lighter cream or white interior; Luster: Dull/earthy exterior, vitreous interior; Structure: Nodular, often spherical or discoid; Specific Gravity: 2.5-2.8

Hardness: 3-4 (Mohs)
Luster: Dull/earthy exterior, vitreous interior

Identified February 17, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of mineral cement within the spaces between sediment particles. These often grow around a nucleus (like a shell or leaf fragment) during diagenesis in sedimentary basins, ranging from millions to hundreds of millions of years old.

Uses & applications

Used primarily as decorative collector specimens, landscape architectural stones, and occasionally sliced and polished for unique jewelry or bookends.

Geological facts

Often mistaken for dinosaur eggs or fossils due to their symmetrical shape. Some concretions, known as Septarian nodules, feature unique internal shrinkage cracks filled with crystals.

Field identification & locations

Identify in the field by their distinct rounded or 'cannonball' shape which contrasts with the surrounding layered sedimentary rock. Commonly found in shale or sandstone cliffs and along coastlines or riverbeds.

Other sedimentary specimens

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Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Sandstone

Arenite (primarily SiO2)

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Shale or Slaty Mudstone

Argillaceous sedimentary rock

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

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Sandstone (Quartz-rich)

Arenite (SiO2)

sedimentary

Sandstone (with man-made markings)

Arenite (composed primarily of Quartz, SiO2)

sedimentary