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I don't know how many times I've been in the field trying to figure out which way is north, how I will fit that gigantic rock in my pack, how to put that gigantic rock in someone else's pack, or something equally as silly, when someone picks up a rock and says, "Hey, look at this calcite!"
I reply with, "That's quartz."
One of us is inevitably wrong, but how do we solve this dilemma?
Quartz and calcite are two common minerals that can look similar, but they have distinct properties that make them easy to tell apart. Here are the key differences:
1. Reaction to Acid
My first reaction is to pull out my acid. Hydrochloric acid, that is. Calcite reacts with hydrochloric acid (HCl), muriatic acid (diluted, less pure HCl) or even vinegar, fizzing or bubbling as it releases CO₂.
Quartz does not react to acid at all.
I purchased these squeeze dropper bottles in 2019 (that's like 7 years ago now, yikes!) to carry a small bottle of acid into the field with me. I'm still using the first one. It has never leaked in my rock bag... yet... and I'm pretty rough with it, since I usually forget it's there until there is a quartz / calcite fiasco.
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| 30 mL Squeezable Eyedropper Bottle with Cap |
2. Hardness (Scratch Test)
Quartz (7 on the Mohs scale) is much harder than calcite (3 on the Mohs scale). If I don't have my rock bag, I pull out my trusty Leatherman Skeletool and run the blade against a piece of the rock in question. A steel knife will easily scratch calcite, but it will not scratch quartz.
If there is any glass laying around that is not your windshield or your eyeglasses, you can see if the rock scratches it. Quartz can scratch glass, while calcite cannot.
3. Cleavage and Fracture
When all else fails, hit it with your rock hammer! Calcite has perfect rhombohedral cleavage, meaning it breaks into three planes ( not necessarily 3 pieces) at angles that aren’t 90 degrees.
Quartz has no cleavage and instead exhibits conchoidal fracture, meaning it breaks in curved, shell-like patterns.
4. Crystal Shape
This should probably be the first step, but if you're like me, you probably don't do things in order, or the smart way. (The hard way is the only way, right?) Look at the rock!
If you see any well formed crystals, look at the shape. Quartz typically forms six-sided (hexagonal) crystals with pointed terminations. Calcite often forms rhombohedral or scalenohedral (dog-tooth) crystals.
5. Transparency and Luster
Again, look at the rock. Like really study it...
Both minerals can be transparent to opaque, but calcite often has a pearly to vitreous (glassy) luster, while quartz is usually just vitreous.
Some clear calcite specimens exhibit double refraction, meaning objects viewed through them may appear doubled. Quartz does not show this effect.
6. Color and Varieties
This one is low on the list, because color seems like the most obvious thing to look at, but it never actually helps me. If I'm confused about a rock being quartz or calcite, it's probably white or clear-ish.
Quartz comes in many colors (clear, white, purple, pink, yellow etc.), but so does calcite (clear, white, yellow, green, blue, red, or brown), and vary rarely am I finding colorful varieties of either one. If you are, please take me with you on your next adventure!
7. UV Reactivity
Does it glow under a UV flashlight? Not all calcite will fluoresce, but some does, and though it can be many colors due to impurities, it will usually be a red or orange color.
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| Calcite glowing orange under 365 nm UV light |
Quick Field Test Summary
Test with acid: Calcite fizzes; quartz doesn't.
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Check hardness: Quartz won’t scratch easily, while calcite scratches with a knife.
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Look at how it breaks: Calcite cleaves into angled planes; quartz breaks with curved, shell-like fractures.
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Examine crystal shape: Quartz forms hexagonal points; calcite forms rhombs or dog-tooth crystals.
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Watch for optical clues: Double refraction or pearly luster points to calcite.
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Don’t rely on color alone: Both minerals come in many shades.
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Use UV light if you have it: Some calcite fluoresces; quartz usually doesn’t.
By using these tests, you can confidently distinguish between quartz and calcite!
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