sedimentary
Septarian Nodule (or Mudstone with Septarian Cracking)
Septarian Concretion (Composition: Calcite, Aragonite, and Limestone/Claystone)
Hardness: 3.5-4 (Mohs scale); Color: Tan, grey, or light brown exterior; Luster: Dull/Earthy on the outside, Vitreous inside cracks; Crystal structure: Trigonal (calcite); Cleavage: Perfect rhombohedral in calcite veins; Specific gravity: approx. 2.7.
- Hardness
- 3
- Color
- Tan, grey, or light brown exterior
- Luster
- Dull/Earthy on the outside, Vitreous inside cracks
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Physical properties
Hardness: 3.5-4 (Mohs scale); Color: Tan, grey, or light brown exterior; Luster: Dull/Earthy on the outside, Vitreous inside cracks; Crystal structure: Trigonal (calcite); Cleavage: Perfect rhombohedral in calcite veins; Specific gravity: approx. 2.7.
Formation & geological history
Formed during the Cretaceous period (50-70 million years ago) from mud balls that dried and shrunk, creating cracks (septaria). These cracks later filled with minerals like calcite or aragonite precipitating from groundwater.
Uses & applications
Used primarily for ornamental purposes like lapidary work, cabochons, bookends, and as educational geological display specimens.
Geological facts
The name 'septarian' comes from the Latin word 'septum' (partition), referring to the cracks that divide the concretion. Some cultures believe they possess metaphysical healing properties, though this is not supported by science.
Field identification & locations
Identify by the characteristic 'turtle back' or mosaic-like pattern of angular cracks on the surface. Found globally in ancient seabed sediments; notable locations include Utah (USA), Madagascar, and various UK coastlines.
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