sedimentary

Septarian Nodule

Septarian Concretion (Calcite, Aragonite, and Limestone)

AI-generated identification · may be incorrect

Hardness varies by mineral (calcite is 3, limestone is around 3-4); colors include yellow, brown, grey, and white; dull to vitreous luster; massive concretionary structure with radial and concentric shrinkage cracks filled with secondary minerals; specific gravity around 2.6-2.7.

Identified August 1, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in sedimentary environments roughly 50 to 100+ million years ago during the Cretaceous period. Mud or sediment accumulated around a shell or organic core, dried, cracked, and was later filled with mineral-rich groundwater precipitating calcite and aragonite.

Uses & applications

Popular as ornamental lapidary material, carved into spheres or animal figures, polished as display specimens, and collected by mineral enthusiasts.

Geological facts

The word septarian comes from the Latin word 'septum', meaning partition, referring to the characteristic cracks or partitions inside the nodule. Sometimes mistaken for fossilized turtle shells by amateur collectors.

Field identification & locations

Easily identified by its distinctive turtle-shell-like pattern of intersecting mineral-filled veins on the exterior or when cut open. Commonly found in marine sedimentary layers, such as the Pierre Shale in the United States and deposits in Madagascar and Utah.