RocksdayCalcite vs Quartz
How to tell them apart
One of the most common minerals on Earth and the main ingredient of limestone and marble. It breaks into perfect slanted boxes (rhombs) and fizzes in acid.
Photo: Robert M. Lavinsky, CC BY-SA 3.0
The second most abundant mineral in Earth's crust. Clear crystals grow as six-sided prisms capped by a pointed termination, and quartz shows up as grains in sand, granite and countless other rocks.
Photo: Didier Descouens, CC BY-SA 4.0
Quick tests
- Scratch glass: Quartz scratches glass (Mohs 7); Calcite can't (Mohs 3).
- Acid (vinegar or dilute HCl): Calcite fizzes; Quartz doesn't.
Key differences
- Quartz is far harder, has no cleavage and does not fizz.
- Calcite is soft enough to scratch with a knife, fizzes in acid and breaks into rhombs.
Side by side
| Property | Calcite | Quartz |
|---|---|---|
| Type | Mineral | Mineral |
| Mohs hardness | 3 | 7 |
| Luster | Vitreous | Vitreous |
| Streak | White | White |
| Colors | Clear, White, Orange, Yellow, Blue, Green | Clear, White, Gray |
| Crystal system | Trigonal | Trigonal |
| Specific gravity | 2.71 | 2.65 |
| Formula | CaCO₃ | SiO₂ |
| Found in | Iceland; Chihuahua, Mexico | Hot Springs, Arkansas; Minas Gerais, Brazil |
Properties that are the same for both are grayed out.
How to identify calcite
- Fizzes in cold dilute hydrochloric acid (or slowly in vinegar).
- Breaks into rhomb-shaped pieces along three cleavage planes.
- Hardness 3 — a copper coin barely scratches it, a knife easily does.
- Clear pieces show double images of text underneath.
Crystal system: Trigonal
Full Calcite guide →How to identify quartz
- Scratches glass easily and can't be scratched by a steel knife.
- Six-sided prisms with pointed ends when it forms crystals.
- Breaks with curved, shell-like (conchoidal) fractures — no flat cleavage planes.
- Does not fizz in vinegar or dilute acid.
Crystal system: Trigonal
Full Quartz guide →More comparisons
Could you tell them apart from a blurry close-up?
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