sedimentary

Polished septarian nodule (likely)

Septarian calcite–aragonite nodule with iron-oxide-rich matrix

AI-generated identification · may be incorrect

Hardness is typically 3–4 for calcite/aragonite, with harder iron oxides around 5–6; cream, brown, and reddish-brown coloration, dull to waxy polished luster, and irregular internal fracture patterns. The rounded, polished form and contrasting mineral-filled cracks are characteristic.

Identified August 29, 2026More sedimentary →

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Physical properties

Formation & geological history

Septarian nodules formed when mineral-rich sediments compacted around a carbonate-rich concretion, later developing shrinkage cracks filled by calcite, aragonite, or iron oxides. They commonly formed in marine sediments during the Cretaceous to Tertiary, although age varies by locality.

Uses & applications

Used as ornamental tumbled stone, cabochons, display specimens, and occasionally decorative carvings; it has modest jewelry and collector demand.

Geological facts

The cream material is usually calcite or aragonite, while red-brown areas commonly contain hematite or limonite. Septarian nodules are often called “dragon stone,” though that trade name can refer to several unrelated materials.

Field identification & locations

Identify by its rounded concretion shape, cream carbonate matrix, brown iron-oxide patches, and vein-like or cavity-filled patterns; common sources include Madagascar, Morocco, Utah, and Texas. A mineral-acid test may fizz on exposed calcite, but avoid damaging polished pieces.