sedimentary
Septarian Nodule
Septarian Concretion
Hardness: 3.5-4 (Mohs), Color: Brown, grey, yellow, tan, Luster: Dull to earthy (matrix) / vitreous (calcite veins), Structure: Massive with distinctive angular cracking patterns filled with crystals.
- Hardness
- 3
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Physical properties
Hardness: 3.5-4 (Mohs), Color: Brown, grey, yellow, tan, Luster: Dull to earthy (matrix) / vitreous (calcite veins), Structure: Massive with distinctive angular cracking patterns filled with crystals.
Formation & geological history
Formed during the Cretaceous period (50-70 million years ago) when volcanic eruptions killed sea life that settled on the ocean floor and formed nodules via chemical precipitation around organic matter. As they dried, they shrank and cracked, and minerals later filled the gaps.
Uses & applications
Primarily used for decorative purposes, ornamental carvings, jewelry (cabochons), and as a popular specimen for mineral collectors.
Geological facts
The name comes from the Latin 'septum' (partition) or 'septem' (seven) because the radial cracks often create seven-sided segments. Famous examples are found in Utah and Madagascar.
Field identification & locations
Identify by looking for 'turtle-back' patterns of intersecting lines or veins of different minerals (usually calcite or aragonite) within a clay-ironstone or limestone matrix.
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