RocksdayConglomerate vs Coquina
How to tell them apart
Rounded pebbles and cobbles glued together by finer sand and cement — a fossil riverbed or beach.
Photo: James St. John, CC BY 2.0
A rock made almost entirely of broken shells loosely cemented together.
Photo: PaleoNeolitic, CC BY 4.0
Quick tests
- Hardness: Conglomerate is clearly harder: Mohs 5–7 versus 2.5–3.5.
- Acid (vinegar or dilute HCl): Coquina fizzes; Conglomerate doesn't.
Key differences
- Conglomerate is made of pebbles, not shells.
Side by side
| Property | Conglomerate | Coquina |
|---|---|---|
| Type | Sedimentary rock | Sedimentary rock |
| Mohs hardness | 5–7 | 2.5–3.5 |
| Luster | Dull | Dull |
| Colors | Brown, Gray, Red | White, Yellow, Brown |
| Crystal system / structure | Clastic, rounded pebbles in matrix | Loosely cemented shell fragments |
| Specific gravity | 2.4–2.7 | 1.8–2.4 |
| Found in | Montserrat, Spain; Kata Tjuta, Australia | St. Augustine, Florida; Shark Bay, Australia |
Properties that are the same for both are grayed out.
How to identify conglomerate
- Contains rounded pebbles larger than 2 mm.
- Pebbles of different rock types.
- Sand or mud fills the gaps.
- Looks like natural concrete.
Structure: Clastic, rounded pebbles in matrix
Full Conglomerate guide →How to identify coquina
- Visible shell pieces throughout.
- Porous and light.
- Fizzes strongly in acid.
Structure: Loosely cemented shell fragments
Full Coquina guide →More comparisons
Could you tell them apart from a blurry close-up?
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