sedimentary

Septarian Nodule (Thunder Egg)

Septarian Concretion (CaCO3 + CaMg(CO3)2 + SiO2)

Hardness: 3.5-4 (calcite portions) to 7 (chalcedony portions); Color: Tan/brown exterior with internal yellow calcite, brown aragonite, and grey limestone; Luster: Dull/earthy exterior, vitreous/pearly interior; Crystal Structure: Trigonal/Orthorhombic; Specific Gravity: 2.6-2.8

Hardness: 3
Luster: Dull/earthy exterior, vitreous/pearly interior

Identified March 29, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed during the Cretaceous period (approx. 50-70 million years ago). They originated as organic masses in seafloors that attracted minerals, forming a hard ball. As the ocean receded, the balls dried and cracked, and were later filled with calcite or aragonite crystals.

Uses & applications

Primarily used as ornamental display pieces, bookends, or sliced into cabochons for jewelry. Many are polished or cut in half ('thunder eggs') to reveal the internal 'star' or 'turtle back' patterns.

Geological facts

Also known as 'Dragon Stones' due to their scaly appearance. The name comes from the Latin word 'septum' meaning partition, referring to the cracks that divide the concretion.

Field identification & locations

Identify by the distinctive 'turtle-shell' pattern of cracks on the surface and a spherical/ovoid shape. Often found in mudstone or shale deposits. Common locations include Utah (USA), Madagascar, and Morocco.

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