The Mohs 3 reference. Fizzes in vinegar, cleaves in three directions, and doubles the image of a line seen through it.
Everywhere. It builds limestone, marble, chalk, stalactites and most shells5 field tests
Calcite is calcium carbonate and it is one of the most abundant minerals on the planet — it makes limestone, marble, chalk, travertine, stalactites and the shells of most marine organisms. If you have picked up a pale, soft, glassy or sugary stone that fizzes, this is almost certainly it.
Its identification is the easiest in this room because it has three unmistakable properties. It is hardness 3, the scale's own reference, so a copper coin scratches it. It effervesces vigorously in dilute acid, and household vinegar is enough. And a clear piece shows double refraction so strongly that a line viewed through it appears twice, which is a genuinely startling thing to see and is the reason Iceland spar was important to the history of optics.
The reason calcite matters for identification is mostly negative: it is the thing a great many hopeful finds turn out to be. Clear calcite gets mistaken for quartz, orange calcite for citrine, banded calcite for agate, and dyed calcite is sold as almost anything. All of it separates in one drop of vinegar.
Calcite is the mineral where the classic field tests work best, and it is worth learning on because the same tests then serve everywhere else. A copper coin, a bottle of vinegar and a pencil line on paper are the entire kit.
The double refraction test only works on transparent material, but where it works it is conclusive: place a clear cleavage fragment over a single sharp line and look down through it. Calcite splits the image into two distinct lines.
| Property | Calcite | What it separates it from |
|---|---|---|
| Mohs hardness | 3 — the scale's reference | Quartz (7); fluorite (4) |
| Reaction to vinegar | Effervesces briskly | Every silicate — quartz, agate, jasper |
| Cleavage | Perfect, three directions, rhombohedral | Quartz, which has none |
| Double refraction | Very strong — a line appears doubled | Almost everything else |
| Specific gravity | 2.71 | Aragonite 2.95; dolomite 2.85 |
| Streak | White | — |
| Habit | Rhombs, scalenohedral 'dogtooth', massive, botryoidal | — |
| Fluorescence | Variable — often red, orange or blue | Suggestive, not diagnostic |
Aragonite and dolomite are the two carbonates most easily confused with calcite. Dolomite fizzes only weakly and mostly when powdered; aragonite has a different cleavage and is denser.
Ordered by how much each one proves. The decisive tests are worth your first two minutes; the supporting ones only matter once those agree.
Calcite effervesces briskly in household vinegar, producing visible fizzing that you can often hear if you hold it close. No silicate does this, so a positive result immediately eliminates quartz, agate, jasper, opal and every feldspar. Dolomite fizzes only sluggishly and mainly when scratched to a powder first, which is the standard way to separate the two.
At hardness 3, calcite is marked by a copper coin at about 3.5, and easily cut by a steel knife. That places it below fluorite and far below quartz. A pale glassy stone that a coin scratches is very unlikely to be anything but a carbonate or a sulphate.
Calcite's birefringence is among the strongest of any common mineral. Lay a clear cleavage rhomb over a single pencil line and look straight down: the line appears as two separate lines. Rotate the fragment and one image travels around the other. This is conclusive, and it is the property that made Iceland spar historically important.
Calcite cleaves perfectly in three directions, producing rhombohedral fragments — leaning boxes with no right angles. Break a piece and the fragments should be little skewed cubes. Quartz produces curved conchoidal chips; fluorite produces triangular octahedral flats; feldspar cleaves in two directions at close to ninety degrees.
Calcite has more crystal habits than almost any mineral — over a thousand forms are recorded. The two most recognisable are the rhomb and the scalenohedron, the latter forming the sharp pointed 'dogtooth spar' crystals that line many cavities. A pointed, six-sided, sharply tapering crystal that a coin scratches is dogtooth calcite, not quartz.
Each of these is checked differently. The ones with their own page have enough that is specific to them to be worth reading separately.
Often sold as citrine. Fizzes in vinegar, which citrine never does.
Read within the checks aboveSpecimen figures rather than gem figures, for material in honest condition. Most finds are worth very little, and saying so plainly is more useful than encouraging you.
| Form | What it typically fetches | Note |
|---|---|---|
| Massive or tumbled calcite | $1 – 5 | Among the commonest minerals on earth |
| Clear Iceland spar rhomb | $5 – 40 | Clarity and size |
| Dogtooth spar cluster | $20 – 150 | Crystal sharpness |
| Fluorescent calcite, named locality | $30 – 400 | Response under UV, plus locality |
| Fine Elmwood or Chinese cabinet specimen | $200 – 2,000+ | Aesthetics and provenance |
Calcite is abundant to the point of being a rock-forming mineral, so specimen value is entirely about crystal quality, colour, fluorescence and locality.
Lovenhall StoneThe tests tell you what it is. The instrument reads a photograph and offers a name to start from — then hands you back to the tile and the coin, which are what settle it.