sedimentary
Septarian Nodule
Septarian Concretion (Calcite, Aragonite, Limestone)
Mohs hardness 3.5-4 (for calcite/aragonite), distinct angular cracks or 'septaria' filled with crystalline minerals (usually yellow calcite and brown aragonite) within a grey/brown limestone matrix, waxy to vitreous luster on crystal portions.
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Physical properties
Mohs hardness 3.5-4 (for calcite/aragonite), distinct angular cracks or 'septaria' filled with crystalline minerals (usually yellow calcite and brown aragonite) within a grey/brown limestone matrix, waxy to vitreous luster on crystal portions.
Formation & geological history
Formed as sedimentary concretions, often around a fossil nucleus, in ancient ocean beds during the Cretaceous period (50-70 million years ago). Drying out caused shrinkage cracks which were later filled by mineral-rich groundwater.
Uses & applications
Commonly tumbled, polished, and carved into spheres, eggs, or pendants for jewelry and decorative collecting.
Geological facts
The name comes from the Latin word 'septum', meaning partition, referring to the cracks/separations in the rock, or 'septem', meaning seven, due to a common misconception about the number of cracks.
Field identification & locations
Easily identified by its 'dragon egg' or turtle shell appearance with distinct polygonal geometric patterns of yellow/brown lines. Found largely in Madagascar, USA (Utah), and New Zealand.
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