sedimentary
Turtle Stone / Septarian Nodule (Fragment)
Septarian Concretion (Calcite, Aragonite, and Limestone)
Mohs hardness of 3.5-4 for the calcite veins, color is greyish-brown with lighter intersecting mineral veins, luster ranges from dull to vitreous in the veins, crystal structure is trigonal/orthorhombic for the filling minerals, specific gravity around 2.6-2.7.
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Physical properties
Mohs hardness of 3.5-4 for the calcite veins, color is greyish-brown with lighter intersecting mineral veins, luster ranges from dull to vitreous in the veins, crystal structure is trigonal/orthorhombic for the filling minerals, specific gravity around 2.6-2.7.
Formation & geological history
Formed in sedimentary mudstone layers during the Cretaceous period (roughly 50 to 100 million years ago) through the process of concretion where mud balls shrank and cracked from drying or gas pressure, and mineral-rich waters subsequently filled the cracks.
Uses & applications
Used primarily in lapidary arts, decorative items, bookends, and as a popular specimen for mineral and fossil collectors.
Geological facts
The characteristic pattern of intersecting veins resembles the shell of a turtle, leading to the folk name 'turtle stone'. The word septarian comes from the Latin word septum, meaning partition, referring to the radiating cracks inside.
Field identification & locations
Easily identified by its distinctive spider-web or geometric fracture pattern filled with lighter-colored minerals like calcite. Often found eroding out of marine shale and clay deposits worldwide.
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