RocksdayConcretion vs Geode
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 hollow, rounded rock whose cavity is lined with crystals such as quartz, amethyst or calcite.
Photo: James St. John, CC BY 2.0
Quick tests
- Hardness: Geode is clearly harder: Mohs 6.5–7 versus 3–6.
Key differences
- Geodes are hollow.
- Concretions are solid throughout.
Side by side
| Property | Concretion | Geode |
|---|---|---|
| Type | Sedimentary rock | Sedimentary rock |
| Mohs hardness | 3–6 | 6.5–7 |
| Luster | Dull | Dull outside, sparkling inside |
| Colors | Brown, Gray, Red | Gray, White, Purple, Brown |
| Crystal system / structure | Mineral-cemented nodule | Hollow nodule lined with crystals |
| Specific gravity | 2.4–3.2 | 2.5–2.7 |
| Found in | Moeraki Boulders, New Zealand; Bowling Ball Beach, California | Keokuk, Iowa; Rio Grande do Sul, Brazil |
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 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 →More comparisons
Could you tell them apart from a blurry close-up?
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