sedimentary

Septarian Nodule

Septaria (composed of Calcite, Aragonite, and Limestone)

Hardness: 3.5-4 (Mohs), Color: Yellow, brown, and grey. Luster: Waxy to earthy. Crystal structure: Trigonal (Calcite) or Orthorhombic (Aragonite). Features distinct radial cracks or 'septa' filled with minerals.

Hardness: 3
Color: Yellow, brown, and grey
Luster: Waxy to earthy

Identified July 1, 2026More sedimentary →

Explore 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

Formed during the Cretaceous period (50-70 million years ago) in marine environments. Volcanic eruptions killed sea life which sank to the bottom; chemical reactions between decaying matter and sediments formed mud balls that later cracked and were filled with mineral deposits.

Uses & applications

Primarily used for lapidary work, home decor, ornamental carvings, and as a popular specimen for rock collectors.

Geological facts

The name 'Septarian' comes from the Latin word 'septum' meaning partition, referring to the cracks that divide the nodule. They are sometimes called 'Dragon Stones' due to their scaly, cracked appearance.

Field identification & locations

Identify by looking for a 'tortoise shell' pattern with yellow calcite centers and grey limestone exteriors. Most commonly found along the Gulf of Mexico and in southern Utah, USA.

Other sedimentary specimens

See all

Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Cone-in-cone structure (Limestone)

Cone-in-cone structure (Secondary Sedimentary Structure)

sedimentary

Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary

Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary