How to tell calcite from quartz
Hardness first: calcite is 3 and quartz 7, four Mohs points apart. Then rhomb cleavage against conchoidal fracture. Density fails: 2.7102 against 2.65 is 2.27%.
To tell calcite from quartz, test hardness on the base: calcite is hardness 3 and a steel point marks it; quartz is 7 and scratches steel and glass. Confirm with the break: calcite cleaves into rhombs, quartz fractures conchoidally. A clear calcite crystal doubles a line seen through it. Density fails: 2.7102 against 2.65.
In short
- Hardness decides, four Mohs points apart. The Handbook of Mineralogy gives calcite 3 and quartz 7. A steel point marks calcite and skates on quartz. Test on the base, never a display face.
- The break confirms it. Calcite has a perfect {10-11} cleavage and breaks into rhombs. Quartz's cleavage is rarely observable and it fractures conchoidally, like glass.
- Clear calcite doubles an image; quartz, for practical purposes, does not. Calcite's birefringence is 0.172 and quartz's 0.009, about 19 times smaller. Calcite ranks 16th and quartz 131st of the 138 reference-set sheets that print two or more refractive indices.
- The scale is the weakest test. Calcite's density is 2.7102 and quartz's 2.65, a 2.27% gap: 0.17 g on a 20.00 g piece, and massive quartz at 2.59 to 2.63 blurs it further.
- Acid is the textbook test, and this page does not use it. None of the 176 Handbook sheets in this site's reference set records effervescence. The standing reason for giving no acid procedure is on how to remove calcite from a specimen.
| Test | Calcite | Quartz | Gap | Does it choose? |
|---|---|---|---|---|
| Hardness (Mohs) | 3 | 7, variable by direction and form | 4 points | Yes, first |
| Cleavage and fracture | {10-11} perfect: rhombs; fracture conchoidal | Cleavage rarely observable; fracture conchoidal | Rhombs or none | Yes, second |
| Refractive indices | 1.486 and 1.658 | 1.544 and 1.553 | - | - |
| Birefringence | 0.172 | 0.009 | About 19 times | Yes, on clear material: a doubled line |
| Density, measured | 2.7102 | 2.65; 2.59 to 2.63 massive | 2.27% | Weakly, and not on massive quartz |
| 20.00 g piece, weight in water | 12.62 g | 12.45 g | 0.17 g | Only on a good scale and solid material |
| Luminescence as recorded | May fluoresce red, blue, yellow and others, SW and LW | May be triboluminescent; piezoelectric, pyroelectric | - | No: may is not a test |
| Effervescence recorded on the sheet | No | No | - | Not from the reference; see the FAQ |
| Chemical formula | CaCO3 | SiO2 | - | Only by analysis |
How to tell calcite from quartz: the hardness-first rule
Calcite is hardness 3 and quartz hardness 7, so a steel point settles most specimens in one stroke. The numbers are the Handbook of Mineralogy's. A steel point, around 5.5 on the everyday scale in our hardness guide, leaves a groove in calcite and slides on quartz, leaving only a grey metal trail that wipes off. Quartz also scratches window glass and calcite does not. Test on the base or a broken surface, never a crystal face, and wipe the mark before you decide; the method and its traps are on how to test mineral hardness properly.
The rule, in the order that loses least: hardness on the base; then look at the break for rhombs; then, if the piece is clear, look through it at a line. Soft, rhombs, doubled line: calcite. Hard, curved glassy fracture, single line: quartz. Density comes last and is usually unnecessary. Both species are covered in the species index, and the calcite-rich localities include Weardale and Alston Moor, both on the calcite sheet's Distribution list.
Does calcite cleave and quartz fracture?
Yes: calcite's {10-11} cleavage is perfect, and quartz's cleavage is, in the Handbook's words, rarely observable. A broken calcite edge shows small stepped rhombs that catch the light together. Quartz breaks with the curved, shell-like conchoidal surface of glass, and its sheet records only a poor cleavage and a rhombohedral parting. The difference between cleavage, parting and fracture on a real specimen is on cleavage or fracture.
Crystal shape helps when there is one: the calcite sheet notes over 1000 forms, crystals to 7 m; the quartz sheet notes over 500, to 6 m and 36 t, as prisms with rhombohedral terminations. Calcite's scalenohedra and rhombs do not look like a six-sided quartz prism, but the massive and drusy forms of both do.
Why clear calcite doubles an image: birefringence 0.172 against 0.009
Calcite's birefringence is about 19 times quartz's, which is why a clear calcite rhomb shows two copies of a line beneath it. The calcite sheet prints refractive indices of 1.658 and 1.486, a difference of 0.172; the quartz sheet prints 1.544 and 1.553, a difference of 0.009. The calcite sheet's Distribution list opens with Iceland, at the Helgustadanáma mine, Reydarfjord.
Where does that sit in the reference set? Across the 176 Handbook sheets, converted with pdftotext -layout and every refractive index after an equals sign read by regular expression, 138 print two or more values. Taking the highest printed index minus the lowest, calcite's 0.172 is 16th; the 15 above it include rhodochrosite, smithsonite, siderite, magnesite and dolomite, all carbonates too. Quartz's 0.009 is 131st. So a strong doubled image is a carbonate sign, not a calcite proof; the aragonite and calcite page separates the carbonates from each other.
Why the scale and the acid bottle come last
Calcite and quartz differ in density by only 2.27%. A 20.00 g piece weighs 12.62 g in water as calcite (2.7102) and 12.45 g as quartz (2.65), 0.17 g apart, and massive quartz at 2.59 to 2.63 moves toward calcite's side. Hardness, cleavage and birefringence all give wider gaps; how to measure specific gravity at home explains the resolution such a gap needs.
We counted what the reference says about acid. The word effervesce contains the ff ligature that pdftotext renders as a stray glyph on older sheets, so the search was ligature-aware and case-insensitive, and its positive control, the same ligature in diffraction and Jefferson, matched 30 sheets including hemimorphite's. The result: 0 of the 176 sheets record effervescence, calcite's included. The reaction is real and textbook, but the primary data reference does not carry it, and this page gives no acid procedure; the reason is on how to remove calcite from a specimen.
Who the answer quartz is wrong for: anyone whose white vein filling is marked by steel. Who the answer calcite is wrong for: anyone whose piece scratches glass, however rhomb-like its shape; quartz can replace calcite and keep its outline, covered on what a pseudomorph is. The rest of the guides are indexed at the collecting guides. Nothing here is offered for sale; the wanted list is the only commercial route.