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A septarium (plural, septaria) is a kind of concretion with a network of cracks filled with coarse mineral crystals. This septarium is filled with calcite and lined with brown siderite. (more below)
Septarian concretion
Photo (c) 2012 Andrew Alden, licensed to About.com (fair use policy)
The cracks in septaria run both concentrically and radially; the radial cracks widen inward. The central void may or may not be filled. Septaria can range in size from tiny to several meters across. They are also commonly called septarian concretions, septarian nodules, turtle stones, and beetle stones. They're popular among lapidarists and other makers of decorative objects.

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