RocksdayConcretion vs Fulgurite
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
Branching tubes of glass formed when lightning strikes sand and melts it instantly.
Photo: John Alan Elson, CC BY-SA 4.0
Quick tests
- Hardness: Fulgurite is clearly harder: Mohs 5.5–6.5 versus 3–6.
- Heft: Concretion feels noticeably heavier for its size (specific gravity 2.4–3.2 vs 1.5–2.2).
- Texture: Concretion is a rock — a mix of grains or minerals; Fulgurite is a single mineraloid.
Key differences
- Concretions are solid, not hollow glass tubes.
Side by side
| Property | Concretion | Fulgurite |
|---|---|---|
| Type | Sedimentary rock | Mineraloid |
| Mohs hardness | 3–6 | 5.5–6.5 |
| Luster | Dull | Glassy inside, sandy outside |
| Colors | Brown, Gray, Red | Brown, Gray, White |
| Crystal system / structure | Mineral-cemented nodule | Amorphous (lightning-fused glass) |
| Specific gravity | 2.4–3.2 | 1.5–2.2 |
| Found in | Moeraki Boulders, New Zealand; Bowling Ball Beach, California | Sahara Desert; Florida beaches |
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 fulgurite
- Hollow, root-like tubes.
- Glassy, bubbly lining; rough sand-coated outside.
- Fragile and light.
Structure: Amorphous (lightning-fused glass)
Full Fulgurite guide →More comparisons
Could you tell them apart from a blurry close-up?
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