sedimentary

Polished Septarian Nodule

Septarian calcite–aragonite nodule, commonly with brown/yellow calcite and dark barite or iron-oxide veins

AI-generated identification · may be incorrect

Hardness about 3–4 (vein minerals may vary); tan-yellow matrix with black branching cracks and vitreous-to-waxy polished luster. The rounded form is likely tumbled or cabochon-polished; crystal structure varies by mineral filling.

Identified September 15, 2026More sedimentary →

Explore Polished Septarian Nodule in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Septarian nodules formed when mineral-rich cavities and cracks developed in muddy marine sediments, commonly during the Cretaceous, roughly 50–100 million years ago. Later groundwater deposited calcite, aragonite, barite, and iron minerals in the fractures.

Uses & applications

Used as a lapidary material for cabochons, spheres, polished stones, and decorative objects; valued mainly for its distinctive web-like pattern.

Geological facts

The name derives from Latin septum, meaning “partition,” referring to the polygonal fracture network. Some specimens are marketed as “dragon stone,” though that trade name is not scientifically specific.

Field identification & locations

Identify it by its rounded nodule form, polygonal dark vein network, and cream-to-brown interior; dilute acid may fizz on exposed calcite. Common sources include Utah and Madagascar.