Quick answer: the mineral most often mistaken for quartz is calcite, followed by white feldspar in rocks, glass in shops, and soft selenite (gypsum). Three quick checks separate them: hardness (a steel nail scratches calcite, feldspar barely, quartz not at all), cleavage (calcite and feldspar break along flat planes, quartz breaks in curves), and vinegar (calcite fizzes, quartz never does).
Suspect #1: Calcite
Clear or white calcite looks remarkably like quartz at a glance, and the two often grow together. They are very different minerals:
- Hardness: calcite is about 3 on the Mohs scale; quartz about 7. A steel nail (around 4.5) scratches calcite but not quartz, per Rock Chasing’s comparison.
- Cleavage: calcite has perfect cleavage in three directions and breaks into rhombus-shaped pieces. Quartz has no cleavage at all.
- Vinegar: calcite fizzes in vinegar as it releases carbon dioxide. Quartz does not react.
- Shape: calcite tends toward blocky rhombs, “dogtooth” points, and skewed-cube shapes; quartz grows six-sided prisms that taper to a point.
- UV: calcite often fluoresces; quartz usually does not.
One more calcite giveaway: clear calcite shows a strong double image when you look through it at printed text, an effect called birefringence (explained in Birefringence: The Optical Trick Behind Double-Image Calcite). Quartz is technically double-refracting too, but so weakly you cannot see it by eye. Compare our Golden Calcite Crystal Point — Ohio or Calcite Crystal with Dark Blade Inclusions — New York with a Clear Quartz Crystal Point and the difference in crystal shape is obvious once you know to look.
Suspect #2: White feldspar
In granite and pegmatite, milky quartz and white feldspar sit side by side. Geology In’s guide gives the tell: feldspar has two cleavage directions meeting at close to 90 degrees, so it shows flat, mirror-like faces that flash as you turn the rock. Quartz breaks with curved, glassy (conchoidal) surfaces and has no cleavage. Quartz also looks glassier and more translucent, while feldspar tends to be duller white, pink, gray, or cream. (Note: on hardness, quartz at 7 scratches glass reliably; feldspar at about 6 generally does not.)
Suspect #3: Glass
Slag glass, beach glass, and molded glass “crystals” are the most common shop-and-roadside impostors. Look for round bubbles, missing growth striations, and a surface that warms quickly in your hand. The full checklist is in our guide Quartz or Glass? 7 Ways to Tell.
Suspect #4: Selenite (gypsum)
Clear selenite blades can look like long quartz crystals, but gypsum is about Mohs 2: your fingernail will scratch it. It also feels warm, splits into thin flexible sheets, and dissolves slowly in water. (More in Selenite: Gypsum’s Glassy, Bendable Form.)
The one-minute field test
- Try to scratch it with a steel nail. No scratch? Likely quartz.
- Look for flat, mirror-like cleavage faces. Present? Calcite or feldspar.
- Put a drop of vinegar on a clean spot. Fizz? Calcite (or another carbonate).
- Look for six-sided prisms and fine horizontal striations. Present? Quartz.
Density will not help much in the field: calcite is about 2.71 and quartz about 2.65, too close to separate by hand.
Quick FAQ
How do I tell calcite from quartz at home?
Scratch it with a steel nail (calcite scratches, quartz does not) and add a drop of vinegar (calcite fizzes, quartz does not). Calcite also breaks into rhombus-shaped fragments.
Is feldspar harder than quartz?
No. Feldspar is about 6 on the Mohs scale and quartz about 7, so quartz scratches feldspar.
Can calcite and quartz grow together?
Yes, very often, in veins and cavities. Their different chemistry is explored in Polymorphs: Aragonite and Calcite.
Sources
- Rock Chasing: Quartz vs. Calcite
- Geology In: Telling Quartz from White Feldspar
- LibreTexts Gemology: Quartz (gem properties)
