sedimentary

Polished septarian nodule

Calcite–aragonite cemented mudstone, commonly with brown limonite/goethite and dark mineral veins

AI-generated identification · may be incorrect

A rounded, polished brown-gray concretion with a waxy to vitreous luster and fine-grained interior; apparent hardness is roughly 3–4 Mohs, though iron-rich areas may be harder. Septarian nodules commonly contain angular calcite-filled cracks and have specific gravity around 2.3–2.7.

Identified August 21, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms when carbonate-rich mud or volcanic ash concretes around a nucleus, dries and cracks, then mineral-rich fluids fill the fractures; most collectible examples are Cretaceous to Paleogene in age. The image is consistent with a polished piece, but mineral identification is not certain without testing.

Uses & applications

Used as decorative stone, cabochons, polished palm stones, and lapidary specimens; not generally used structurally. Value depends on visible septarian pattern, polish, size, and locality.

Geological facts

The name comes from Latin *septum*, meaning partition, referring to the polygonal crack network. Brown coloration commonly reflects iron oxides, while yellow or cream veins are typically calcite.

Field identification & locations

Test an inconspicuous area with vinegar or dilute acid for calcite effervescence; a glass scratch test and close inspection for polygonal veins help distinguish it from jasper or iron-rich stone. Common sources include Madagascar, Utah, Morocco, and England.