RocksdayOrthoclase vs Quartz
How to tell them apart
A potassium feldspar and one of the most common minerals in granite, where it forms the pink or cream-colored blocks.
Photo: Didier Descouens, CC BY-SA 4.0
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
Key differences
- Quartz is glassier, harder and never breaks along flat cleavage planes.
Side by side
| Property | Orthoclase | Quartz |
|---|---|---|
| Type | Mineral | Mineral |
| Mohs hardness | 6 | 7 |
| Luster | Vitreous | Vitreous |
| Streak | White | White |
| Colors | Pink, White, Orange | Clear, White, Gray |
| Crystal system | Monoclinic | Trigonal |
| Specific gravity | 2.55–2.63 | 2.65 |
| Formula | KAlSi₃O₈ | SiO₂ |
| Found in | Granites worldwide; Karlovy Vary, Czech Republic | Hot Springs, Arkansas; Minas Gerais, Brazil |
Properties that are the same for both are grayed out.
How to identify orthoclase
- Blocky pink to cream crystals in granite.
- Two cleavages meeting at a right angle — flat, shiny faces.
- Hardness 6 — scratches glass with effort.
- No fine parallel striations (unlike plagioclase).
Crystal system: Monoclinic
Full Orthoclase 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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