RocksdayGeode vs Thunderegg
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 knobbly rhyolite nodule that, cut open, reveals a star-shaped core of agate or jasper.
Photo: David Rix Eibonvale, CC BY 3.0
Key differences
- Thundereggs are solid with a star-shaped agate core.
- Geodes are hollow with crystal-lined insides.
Side by side
| Property | Geode | Thunderegg |
|---|---|---|
| Type | Sedimentary rock | Igneous rock |
| Mohs hardness | 6.5–7 | 6.5–7 |
| Luster | Dull outside, sparkling inside | Dull outside, waxy inside |
| Colors | Gray, White, Purple, Brown | Brown, Blue, White, Red |
| Crystal system / structure | Hollow nodule lined with crystals | Rhyolite nodule with agate core |
| Specific gravity | 2.5–2.7 | 2.5–2.6 |
| Found in | Keokuk, Iowa; Rio Grande do Sul, Brazil | Central Oregon; Queensland, Australia |
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 thunderegg
- Baseball-sized, bumpy brown nodule.
- Star-shaped agate center when sawed.
- Heavy and solid (unlike hollow geodes).
Structure: Rhyolite nodule with agate core
Full Thunderegg guide →More comparisons
Could you tell them apart from a blurry close-up?
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