A septarian nodule is a rounded concretion with internal cavities or cracks filled by minerals such as calcite or aragonite. Look for the distinct cracked, polygonal surface pattern resembling turtle shell or dragon skin. It occurs in sedimentary shale formations and shows best when sliced and polished.
Is this septarian nodule?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch septarian nodule with a known reference. Septarian Nodule sits at Mohs 3-5, softer than the next harder reference, harder than the previous one.
- 2Read the textureLook at a fresh break with a hand lens. Texture: Concretion of polyhedral blocks separated by mineral-filled radiating cracks (septaria).
- 3Pick out the mineralsComposition: Argillaceous carbonate matrix such as clay ironstone; cracks usually filled with calcite, often iron-bearing. Identify the two or three most abundant grains.
- 4Read the lusterHold the specimen under a strong light. Luster: dull to vitreous.
- 5Match the color rangeCompare against the expected color range: brown, yellow, gray, white.
Often confused with
Septarian Nodule vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside septarian nodule
Minerals reported to co-occur with septarian nodule. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Mohs hardness
- 3-5
- Density
- 2.6-2.9 g/cm³
- Luster
- Dull to vitreous
- Transparency
- Opaque
- Fluorescence
- Often fluorescent under SW UV
- Rarity
- Common
- Uses
- Lapidary, Decorative, Collector
- Host rock
- shale
- Texture
- Concretion of polyhedral blocks separated by mineral-filled radiating cracks (septaria)
- Composition
- Argillaceous carbonate matrix such as clay ironstone; cracks usually filled with calcite, often iron-bearing
Where rockhounds find septarian nodule
6 mapped spotsClassic worldwide localities
- Utah
- Madagascar
- Morocco
- New Zealand
- England
Field-hunting tip
Where it forms: shale. If you start seeing calcite, aragonite, barite in float, you are in the right ground. In the U.S., the densest reported localities are in Kansas, Missouri, Arizona. Start trip planning there.
More septarian nodule photos
Common questions
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Find septarian nodule on the map
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