sedimentary
Septarian Nodule
Septarian Concretion (Calcite, Aragonite, and Limestone)
Hardness of 3-4 on the Mohs scale for the internal minerals (calcite/aragonite) and around 3 for the limestone host; colors range from brown, yellow, and grey to white; dull to vitreous luster; amorphous overall nodular shape with crystalline internal cavities; specific gravity of approximately 2.6 to 2.7.
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Physical properties
Hardness of 3-4 on the Mohs scale for the internal minerals (calcite/aragonite) and around 3 for the limestone host; colors range from brown, yellow, and grey to white; dull to vitreous luster; amorphous overall nodular shape with crystalline internal cavities; specific gravity of approximately 2.6 to 2.7.
Formation & geological history
Formed in sedimentary environments during the Cretaceous period (roughly 50 to 100 million years ago). They begin as muddy sediments containing organic matter that forms concretions, which subsequently crack from shrinkage (syneresis or drying) and are later filled with mineral-rich groundwater precipitating calcite and aragonite.
Uses & applications
Popular in lapidary arts, sliced and polished as decorative display specimens, carved into decorative objects, and collected by mineral enthusiasts.
Geological facts
The name septarian comes from the Latin word 'septum', meaning partition, referring to the distinctive network of angular cavities or cracks dividing the rock into compartments.
Field identification & locations
Easily identified by its distinct turtle-shell-like pattern of intersecting mineral veins cutting through a rounded, brownish sedimentary concretion. Commonly found in locations such as Utah and Madagascar.
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