RocksdayAgate vs Stromatolite
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
Layered mounds built by mats of microbes trapping sediment — among the oldest evidence of life.
Photo: Didier Descouens, CC BY-SA 4.0
Quick tests
- Scratch glass: Agate scratches glass (Mohs 6.5–7); Stromatolite can't (Mohs 3–4).
- Acid (vinegar or dilute HCl): Stromatolite fizzes; Agate doesn't.
- Texture: Stromatolite is a fossil; Agate is a single mineral.
Key differences
- Agate bands are glassy chalcedony, not sediment layers.
Side by side
| Property | Agate | Stromatolite |
|---|---|---|
| Type | Mineral | Fossil |
| Mohs hardness | 6.5–7 | 3–4 |
| Luster | Waxy to vitreous | Dull |
| Streak | White | — |
| Colors | Red, Orange, White, Gray, Brown, Blue | Gray, Brown, White |
| Crystal system / structure | Microcrystalline quartz (chalcedony) | Layered microbial carbonate |
| Specific gravity | 2.58–2.64 | 2.6–2.8 |
| Formula | SiO₂ | — |
| Found in | Lake Superior shores; Rio Grande do Sul, Brazil | Pilbara, Western Australia; Shark Bay, Western 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 stromatolite
- Wavy, domed, cabbage-like layers.
- Fine laminations in cut sections.
- Usually limestone or dolostone; fizzes.
Structure: Layered microbial carbonate
Full Stromatolite guide →More comparisons
Could you tell them apart from a blurry close-up?
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