RocksdayConcretion vs Septarian Nodule
How to tell them apart
A hard, rounded lump formed when minerals cemented sediment around a center.
Photo: James St. John, CC BY-SA 4.0
A clay-rich concretion whose shrinkage cracks filled with yellow calcite.
Photo: James St. John, CC BY 2.0
Quick tests
- Acid (vinegar or dilute HCl): Septarian Nodule fizzes; Concretion doesn't.
Key differences
- Septarians have crystal-filled cracks.
- Plain concretions lack the crystal-filled cracks.
Side by side
| Property | Concretion | Septarian Nodule |
|---|---|---|
| Type | Sedimentary rock | Sedimentary rock |
| Mohs hardness | 3–6 | 3–5 |
| Luster | Dull | Dull with glassy veins |
| Colors | Brown, Gray, Red | Gray, Yellow, Brown |
| Crystal system / structure | Mineral-cemented nodule | Concretion with calcite-filled cracks |
| Specific gravity | 2.4–3.2 | 2.5–2.8 |
| Found in | Moeraki Boulders, New Zealand; Bowling Ball Beach, California | Kanab, Utah; Mahajanga, Madagascar |
Properties that are the same for both are grayed out.
How to identify concretion
- Round, oval or odd-shaped lumps that weather out of softer rock.
- Harder than the surrounding layer.
- Sometimes contain a fossil at the center.
Structure: Mineral-cemented nodule
Full Concretion guide →How to identify septarian nodule
- Gray body crossed by yellow-brown calcite veins.
- Veins wider in the center, thinning outward.
- Fizzes in acid.
Structure: Concretion with calcite-filled cracks
Full Septarian Nodule guide →More comparisons
Could you tell them apart from a blurry close-up?
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