sedimentary

Polished septarian nodule cabochon

Septarian calcite–aragonite nodule, commonly in a clay-rich limestone/marl matrix

AI-generated identification · may be incorrect

Typically Mohs 3–4: dark gray-brown matrix with purple, yellow, or cream mineral-filled cracks and a glassy-to-waxy polished luster. The visible golden radiating crystal is likely calcite; septarian cavities may contain calcite, aragonite, or barite.

Identified September 18, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms when carbonate-rich concretions develop polygonal drying or shrinkage cracks in marine mud, later filled by minerals from groundwater. Many commercial specimens formed during the Cretaceous–Eocene, roughly 50–150 million years ago.

Uses & applications

Used as decorative cabochons, jewelry, lapidary material, and collectible display stones; too soft for high-wear jewelry.

Geological facts

The name derives from Latin septum, meaning “partition,” referring to the crack network. Purple coloration may reflect manganese-bearing minerals or associated volcanic-derived material.

Field identification & locations

Identify by the distinctive web of contrasting mineral veins enclosing a rounded nodule, rather than uniform banding. Common sources include Utah and Madagascar; inspect veins with a loupe and avoid acids, which can etch calcite.