sedimentary

Septarian Nodule

Septarian Concretion (CaCO3 with Calcite and Aragonite)

Hardness: 3.5-4; Color: Gray (mudstone), Yellow (calcite), Brown (aragonite); Luster: Adamantine to dull; Structure: Trigonal crystal system in veins; Cleavage: Perfect rhombohedral in calcite portions; Specific Gravity: 2.6-2.8.

Hardness: 3
Color: Gray (mudstone), Yellow (calcite), Brown (aragonite)
Luster: Adamantine to dull

Identified June 13, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed during the Cretaceous period (approx. 50-70 million years ago). They began as sedimentary slime balls on the ocean floor that cracked as they dehydrated; these cracks (septaria) were later filled with minerals like calcite and aragonite.

Uses & applications

Primarily used for decorative purposes, including polished spheres, bookends, jewelry, and as highly prized cabinet specimens for collectors.

Geological facts

The name 'septarian' comes from the Latin word 'septum' meaning wall or enclosure, and 'septem' meaning seven, referring to the cracks that divide the nodule. Some large nodules found in New Zealand are known as the Moeraki Boulders.

Field identification & locations

Identified by the distinct 'turtle shell' pattern of dark brown or yellow veins cutting through gray sedimentary rock. Commonly found in Utah (USA), Madagascar, and Morocco. Look for round, dull gray boulders that may show crystalline interiors when cracked.

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