sedimentary

Polished septarian nodule

Septarian concretion; predominantly calcite and aragonite cement with clay-rich limestone/mudstone matrix

AI-generated identification · may be incorrect

Typically Mohs 3–4 (calcite-rich areas), brown to gray matrix with cream calcite vein networks, earthy to waxy luster, opaque, and massive rather than crystalline. The polished surface shows contrasting angular internal cracks filled by pale minerals.

Identified August 28, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms when shrinkage cracks develop in carbonate-rich mud concretions, later filled by calcite, aragonite, and sometimes barite or celestine. Many commercial examples formed during the Cretaceous–Tertiary interval, roughly 50–70 million years ago, especially in marine sediments.

Uses & applications

Used as ornamental stone, polished décor, cabochons, bookends, and collector specimens; generally unsuitable for outdoor construction because calcite is soft and acid-sensitive.

Geological facts

The name derives from Latin *septum*, meaning partition, referring to the vein-separated pattern. Cream veins fizz with dilute hydrochloric acid if calcite is present.

Field identification & locations

Identify by a rounded nodule with polygonal pale veins cutting a darker mudstone matrix; common sources include Utah and Madagascar. Avoid vinegar or acid testing on valuable polished surfaces.