sedimentary

Polished septarian nodule

Septarian concretion: calcite/aragonite veins in a clay-rich carbonate matrix, commonly with barite or iron oxides

AI-generated identification · may be incorrect

Typically Mohs 3–4 overall (calcite 3; aragonite 3.5–4), brown to gray matrix with cream or golden vein fillings, waxy to vitreous polished luster, and irregular polygonal internal cracks; specific gravity about 2.3–2.7.

Identified September 21, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed when mud-rich carbonate concretions developed drying and contraction cracks that later filled with calcite or aragonite. Many commercial specimens formed in the Cretaceous–Eocene, roughly 50–100 million years ago.

Uses & applications

Used as ornamental cabochons, polished slabs, décor, and lapidary material; generally not suitable for high-wear jewelry because carbonate minerals are relatively soft.

Geological facts

The polygonal vein network resembles a cracked turtle shell, giving septarian its name and distinctive appearance.

Field identification & locations

Identify it by dark concretionary matrix crossed by contrasting cream calcite veins, often with a rounded nodule outline and polished surface; common sources include Utah and Madagascar. Avoid acids, harsh cleaners, and prolonged moisture, which can etch carbonate veins.