sedimentary

Septarian Nodule (Thunder Egg variant)

Septarian concretion (Calcite, Aragonite, and Limestone)

Hardness: 3.5-4.0 (Mohs scale); Color: Black or dark grey exterior with yellow to honey-colored calcite interior; Luster: Vitreous to dull; Crystal structure: Trigonal (calcite) and Orthorhombic (aragonite); Cleavage: Perfect rhombohedral in interior crystals.

Hardness: 3
Luster: Vitreous to dull

Identified June 18, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed during the Cretaceous period (approx. 50-70 million years ago) in marine environments. Mud balls formed and cracked due to dehydration or seismic activity, and minerals like calcite precipitated into the cracks from groundwater.

Uses & applications

Primarily used as ornamental display pieces, bookends, jewelry (cabochons), and highly valued by mineral collectors for their unique 'dragon skin' patterns.

Geological facts

Also known as 'Dragon Stones,' they are unique because they are a combination of different minerals working together to create a distinct pattern that is never exactly the same twice.

Field identification & locations

Identify by looking for a smooth, rounded gray exterior that reveals yellow crystalline veins or cavities when cracked or wet. Frequently found in Utah, USA, and Madagascar. Collectors should look for distinct, sharp color contrasts.

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