sedimentary
Septarian Nodule
Septarian Concretion (Calcite, Aragonite, Limestone, and Barite)
Hardness varies by mineral (calcite is 3, limestone is 3-4, aragonite is 3.5-4); brownish, grey, yellow, and black colors; dull to vitreous luster; massive to cryptocrystalline structure; conchoidal to uneven fracture; specific gravity around 2.6-2.7
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Physical properties
Hardness varies by mineral (calcite is 3, limestone is 3-4, aragonite is 3.5-4); brownish, grey, yellow, and black colors; dull to vitreous luster; massive to cryptocrystalline structure; conchoidal to uneven fracture; specific gravity around 2.6-2.7
Formation & geological history
Formed in sedimentary muds or clay beds approximately 50 to 100 million years ago during the Cretaceous period, where mudballs shrank and cracked, and mineral-rich groundwater filled the cavities with calcite, aragonite, and sometimes barite
Uses & applications
Popular for lapidary work, making decorative spheres, bookends, cabochons for jewelry, and highly prized by mineral collectors
Geological facts
The word septarian comes from the Latin word septem, meaning seven, referring to the septarian cracks that typically divide the concretion into seven or more polygonal segments
Field identification & locations
Easily identified by its distinct spiderweb or turtle-shell pattern of dark and light mineral veins crisscrossing a brownish concretionary matrix; commonly found in marine sedimentary layers worldwide, notably in Madagascar and the United States
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