sedimentary
Septarian Nodule (Galt)
Calcareous concretion / Septarian concretion
Hardness: 3-4 (on Mohs scale); Color: Externally drab grey or brown, internally often contains yellow calcite and brown aragonite; Luster: Dull/Earthy (external), Vitreous (internal); Crystal structure: Trigonal (internal calcite); Specific gravity: Approx 2.6-2.8.
- Hardness
- 3-4 (on Mohs scale)
- Luster
- Dull/Earthy (external), Vitreous (internal)
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Physical properties
Hardness: 3-4 (on Mohs scale); Color: Externally drab grey or brown, internally often contains yellow calcite and brown aragonite; Luster: Dull/Earthy (external), Vitreous (internal); Crystal structure: Trigonal (internal calcite); Specific gravity: Approx 2.6-2.8.
Formation & geological history
Formed as concretions in late Cretaceous seabed sediments approximately 50 to 70 million years ago. As the clay-rich mud dried and shrank, cracks formed (septaria) which were later filled by minerals precipitating from groundwater.
Uses & applications
Used primarily as decorative lapidary material, for carving into eggs/spheres, and as ornamental display specimens for collectors.
Geological facts
The name 'Septarian' comes from the Latin word 'septum', meaning partition, referring to the cracks and separations within the rock. They are sometimes called 'Dragon Stones' due to the patterns resembling dragon skin.
Field identification & locations
Identify in the field by their rounded, potato-like exterior and 'cracked' surface texture. Commonly found in riverbeds or eroded cliffs in regions like Utah (USA) or Madagascar. Collectors look for the characteristic pattern of calcite-filled veins.
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