RocksdayAgate vs Thunderegg
How to tell them apart
Banded chalcedony — quartz made of microscopic crystals — that fills cavities in volcanic rock layer by layer. Light passes through the bands when held up to the sun.
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
Quick tests
- Texture: Thunderegg is a rock — a mix of grains or minerals; Agate is a single mineral.
Key differences
- Agate is the filling; the thunderegg is the whole nodule.
Side by side
| Property | Agate | Thunderegg |
|---|---|---|
| Type | Mineral | Igneous rock |
| Mohs hardness | 6.5–7 | 6.5–7 |
| Luster | Waxy to vitreous | Dull outside, waxy inside |
| Streak | White | — |
| Colors | Red, Orange, White, Gray, Brown, Blue | Brown, Blue, White, Red |
| Crystal system / structure | Microcrystalline quartz (chalcedony) | Rhyolite nodule with agate core |
| Specific gravity | 2.58–2.64 | 2.5–2.6 |
| Formula | SiO₂ | — |
| Found in | Lake Superior shores; Rio Grande do Sul, Brazil | Central Oregon; Queensland, Australia |
Properties that are the same for both are grayed out.
How to identify agate
- Concentric or parallel bands, often in red, orange and white.
- Translucent at the edges when backlit.
- Waxy surface on rough, weathered pieces ('pitted' rind).
- Hard enough to scratch glass.
Crystal system: Microcrystalline quartz (chalcedony)
Full Agate 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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