RocksdayAgate vs Chert
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 hard sedimentary rock of microscopic quartz. Dark nodules in chalk are called flint.
Photo: Unknown, Public domain
Quick tests
- Texture: Chert is a rock — a mix of grains or minerals; Agate is a single mineral.
Key differences
- Chert is opaque and lacks agate's neat concentric banding.
Side by side
| Property | Agate | Chert |
|---|---|---|
| Type | Mineral | Sedimentary rock |
| Mohs hardness | 6.5–7 | 6.5–7 |
| Luster | Waxy to vitreous | Waxy to dull |
| Streak | White | — |
| Colors | Red, Orange, White, Gray, Brown, Blue | Gray, Black, Brown, White |
| Crystal system / structure | Microcrystalline quartz (chalcedony) | Microcrystalline quartz |
| Specific gravity | 2.58–2.64 | 2.5–2.7 |
| Formula | SiO₂ | — |
| Found in | Lake Superior shores; Rio Grande do Sul, Brazil | Flint Ridge, Ohio; Alibates, Texas |
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 chert
- Breaks with smooth, curved fractures and sharp edges.
- Waxy luster on fresh breaks.
- Scratches glass.
- Sparks when struck against steel.
Structure: Microcrystalline quartz
Full Chert guide →More comparisons
Could you tell them apart from a blurry close-up?
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