RocksdayGeode vs Septarian Nodule
How to tell them apart
A hollow, rounded rock whose cavity is lined with crystals such as quartz, amethyst or calcite.
Photo: James St. John, CC BY 2.0
A clay-rich concretion whose shrinkage cracks filled with yellow calcite.
Photo: James St. John, CC BY 2.0
Quick tests
- Scratch glass: Geode scratches glass (Mohs 6.5–7); Septarian Nodule can't (Mohs 3–5).
- Acid (vinegar or dilute HCl): Septarian Nodule fizzes; Geode doesn't.
Key differences
- Geodes are hollow and crystal-lined.
Side by side
| Property | Geode | Septarian Nodule |
|---|---|---|
| Type | Sedimentary rock | Sedimentary rock |
| Mohs hardness | 6.5–7 | 3–5 |
| Luster | Dull outside, sparkling inside | Dull with glassy veins |
| Colors | Gray, White, Purple, Brown | Gray, Yellow, Brown |
| Crystal system / structure | Hollow nodule lined with crystals | Concretion with calcite-filled cracks |
| Specific gravity | 2.5–2.7 | 2.5–2.8 |
| Found in | Keokuk, Iowa; Rio Grande do Sul, Brazil | Kanab, Utah; Mahajanga, Madagascar |
Properties that are the same for both are grayed out.
How to identify geode
- Round, lumpy, cauliflower-like outside.
- Feels light for its size (it's hollow).
- Crystals or agate bands inside when broken.
Structure: Hollow nodule lined with crystals
Full Geode 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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