The RockLens field guide
Quartz vs calcite: how to tell them apart
Start with hardness: quartz is Mohs 7 and calcite is Mohs 3. Calcite also breaks along flat cleavage planes; quartz has no cleavage. Both can look clear and glassy, so color and shine won’t distinguish them. See the University of Minnesota’s calcite reference and USGS mineral gemstone reference.
Quartz vs calcite at a glance
| Clue | Quartz | Calcite |
|---|---|---|
| Mohs hardness | 7 | 3 |
| Cleavage | Absent | Perfect rhombohedral cleavage in three directions |
| Dilute acid | No characteristic fizz | Effervesces |
| Glassy luster | Possible | Possible |
See the University of Minnesota’s comparison for more about cleavage and acid response.
Look at the specimen first
- Record where it came from. Keep the collection location or seller’s label with the sample. Those details can help you follow up on an identification.
- Examine existing surfaces in good light. Repeated flat breaks can show cleavage, though a shiny crystal face can look similar. Our guide to crystal faces and cleavage shows how to examine those surfaces without breaking the specimen.
- Look through a clear piece. Transparent calcite shows double refraction: a pencil line seen through it appears doubled, according to the Missouri Geological Survey.
- Check which part you’re examining. A rock can contain several minerals. A test on one grain tells you about that grain; other parts may behave differently.
For example, the university notes that limestone can contain minerals besides calcite. Look at surfaces that are already exposed rather than breaking your specimen.
How hardness helps
Mohs hardness compares resistance to scratching; it does not measure resistance to breaking. The USGS hardness scale gives a reference for the comparison. Our guide to scratch tests covers how to record a result and where the numbers stop being useful. If a reliable test puts your sample near Mohs 3, it is too soft to be solid quartz.
For an ordinary, unmounted sample, follow a hardness kit’s instructions on a small, inconspicuous spot. Wipe away any powder and check for a groove. Leave jewelry, polished keepsakes, coated surfaces and valuable specimens unmarked. Stop if the result is unclear.
Do you need an acid test?
Start with the existing surfaces and, where appropriate, hardness. Calcite reacts with dilute acid, but household vinegar is a weak acid: a University of Florida extension guide says it produces a weak fizz with calcite. For help with further testing, ask a geology educator or mineral club.
Can you identify it from a photo?
A photo can show surface details and suggest a match, but hardness needs a physical test. Save pictures from several angles with your observations. If neither quartz nor calcite fits what you see, consider other minerals.
If you suspect glass, read quartz vs glass. For a valuable stone or a purchase dispute, get an in-person identification.