Quartz vs Calcite: How to Tell Them Apart
The quick answer
A drop of vinegar fizzes on calcite and does nothing on quartz, and a steel knife scratches calcite but never quartz.
Quartz vs calcite is the classic mineral test, and two quick checks settle it almost every time. Calcite is soft at hardness 3, so a steel knife or even a copper coin marks it, and it fizzes when you touch a drop of weak acid like vinegar to it. Quartz is hard at 7, shrugs off a knife, and stays completely inert in acid.
There is a striking optical bonus, too. A clear calcite crystal shows double refraction, splitting text placed underneath into two overlapping images. Clear quartz never does this. Between hardness, the acid fizz, and the double image, you can name the right mineral with household tools.
| Property | Quartz | Calcite |
|---|---|---|
| Transparency | Transparent to opaque | Transparent to opaque |
| Pattern | Six-sided prisms, no cleavage; conchoidal fracture | Rhombohedral cleavage; breaks into slanted blocks |
| Hardness (Mohs) | 7 (knife will not scratch it) | 3 (a knife or nail scratches it easily) |
| Luster | Glassy (vitreous) | Vitreous to pearly on cleavage faces |
| Composition | Silicon dioxide (SiO2) | Calcium carbonate (CaCO3) |
| Price | Affordable; clear quartz is cheap, fine specimens cost more | Affordable; most specimens are inexpensive |
How to tell them apart
Run the acid test first because it is decisive. Place a single drop of white vinegar on an inconspicuous spot. Calcite is calcium carbonate, so it reacts and fizzes, releasing tiny bubbles of carbon dioxide. Quartz is silica and does nothing at all. A fresh, scratched, or powdered surface fizzes most clearly.
Back it up with hardness. Calcite at Mohs 3 is scratched by a steel knife, a nail, and even a copper penny. Quartz at Mohs 7 is much harder than steel, so a knife slides off and leaves no mark, while quartz itself will scratch glass. If you have a clear, transparent piece, the optical test clinches it: set a single calcite rhomb over a line of text and you will see the writing doubled, because calcite splits light into two rays. Quartz produces only one image.
- Acid test: a drop of vinegar fizzes on calcite, never on quartz.
- Hardness: a steel knife scratches calcite (3) but cannot scratch quartz (7).
- Glass test: quartz scratches glass; calcite does not.
- Cleavage: calcite splits into slanted rhombs along flat planes, quartz fractures in curved, shell-like chips.
- Double refraction: clear calcite doubles text seen through it, clear quartz shows a single image.
What each one is
Quartz is silicon dioxide, one of the most abundant minerals in the crust. It crystallizes as hard, six-sided prisms ending in points and has no cleavage, so when it breaks it leaves smooth, curved, shell-like surfaces. Its hardness of 7 and chemical toughness are why quartz survives in sand and gravel long after softer minerals wear away.
Calcite is calcium carbonate and is just as common but completely different in behavior. It is soft, only 3 on the Mohs scale, and it has perfect rhombohedral cleavage, meaning it breaks into tidy slanted blocks along three directions. Calcite builds limestone and marble, dissolves in mild acid, and forms the famous transparent Iceland spar prized for its strong double refraction.
Value & uses
Both minerals are common and inexpensive, so neither is bought for rarity in everyday specimens. Clear quartz points, amethyst, and citrine are sold cheaply as tumbled stones and clusters, with prices rising only for large, flawless, or unusual pieces. Calcite specimens, including the optical Iceland spar and brightly fluorescent varieties, are likewise affordable and popular with collectors and classrooms.
Their uses split along their properties. Quartz, being hard and chemically stable, ends up in glass, abrasives, electronics, and durable jewelry. Calcite, being soft and reactive, is mined as limestone for cement, lime, and agriculture, and its optical form was historically used in instruments. If you have a clear crystal and cannot remember which is which, photograph it and run it through our crystal identifier, then confirm with the vinegar and knife tests at home.
Frequently asked questions
What is the easiest test to tell quartz from calcite?
The acid test. Put a drop of vinegar on the stone: calcite fizzes because it is calcium carbonate, while quartz does nothing. Pair it with a scratch test, since a steel knife marks soft calcite but cannot scratch hard quartz.
Does calcite fizz in vinegar but quartz does not?
Yes. Calcite reacts with weak acids and releases carbon dioxide bubbles, so it visibly fizzes in vinegar. Quartz is silica and is chemically inert in vinegar, so it shows no reaction at all. This is the single most reliable way to separate them.
What is double refraction and which one shows it?
Double refraction is when a clear crystal splits light into two rays, making text underneath appear doubled. Clear calcite, such as Iceland spar, shows this strongly. Clear quartz does not double the image, so a doubled view points straight to calcite.
Is quartz harder than calcite?
Much harder. Quartz is 7 on the Mohs scale and calcite is only 3. A steel knife scratches calcite easily but slides off quartz without leaving a mark, and quartz will scratch glass while calcite will not.
Can quartz and calcite look the same?
They can, especially as clear or white masses. Color and shine alone will fool you, which is why you should rely on the acid fizz, the scratch test, and double refraction. Those three checks tell quartz and calcite apart even when they look identical.
In the field guide
Last updated 2026-06-26. Educational comparison — confirm an identification with the tests described or a qualified expert before relying on it.