sedimentary
Septarian Nodule Segment (Turtle Stone)
Septarian Concretion (CaCO3 with Siderite or Calcite infill)
Hardness: 3.5-4.0 (Mohs scale); Color: Creamy grey to tan exterior with tan/brown cracks; Luster: Dull/earthy exterior; Structure: Microcrystalline with distinct angular shrinkage cracks (septaria).
- Hardness
- 3
- Color
- Creamy grey to tan exterior with tan/brown cracks
- Luster
- Dull/earthy exterior
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Physical properties
Hardness: 3.5-4.0 (Mohs scale); Color: Creamy grey to tan exterior with tan/brown cracks; Luster: Dull/earthy exterior; Structure: Microcrystalline with distinct angular shrinkage cracks (septaria).
Formation & geological history
Formed during the Cretaceous period (approx. 50-70 million years ago) in marine environments. They began as mud balls that cracked as they dehydrated; mineral-rich water then filled the cracks with calcite or siderite.
Uses & applications
Used primarily as decorative specimens, lapidary material for cabochons, and unique collector pieces. Large nodules are often sliced and polished for display.
Geological facts
Also known as 'Dragon Stones' or 'Turtle Stones' due to the shell-like pattern created by the cracks. They are common in areas that were once prehistoric seafloors.
Field identification & locations
Identify in the field by the distinct 'network' of raised or recessed cracks on a rounded, heavy stone. Commonly found in Utah (USA), Madagascar, and various dry lake beds or coastal cliffs. Collectors should look for nodules with high contrast between the host rock and the vein fill.
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