sedimentary

Polished septarian nodule slice

Septarian concretion, commonly calcite/aragonite veins in a limestone–mudstone matrix

AI-generated identification · may be incorrect

Typically Mohs 3–4 overall: brown/yellow limonite or calcite-rich matrix with dark gray-brown carbonate mudstone and pale calcite or aragonite crack fillings; earthy to waxy luster, irregular granular structure, and no consistent cleavage in the whole specimen.

Identified September 3, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms when shrinkage cracks develop in a carbonate-rich mud or concretion and later fill with calcite, aragonite, and iron oxides. Many commercial examples formed in Cretaceous–Eocene marine sediments, roughly 50–100 million years old.

Uses & applications

Primarily a decorative and lapidary specimen, used for polished pieces, cabochons, ornaments, and collections; it has little industrial value.

Geological facts

The name derives from the Latin *septum*, meaning partition, referring to the polygonal internal cracks. Brown coloration commonly reflects iron-oxide staining.

Field identification & locations

Identify it by its flattened nodule form, contrasting dark matrix, rusty brown outer rind, and network of mineral-filled fractures; common sources include Utah, Madagascar, Morocco, and England. Avoid acids and prolonged soaking, which can damage carbonate veins.