Calcite Identification Guide
How to identify calcite by its rhombohedral cleavage, vigorous acid fizz, Mohs-3 hardness, and double refraction, with tests versus quartz and dolomite.
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What Calcite Looks Like
Calcite (calcium carbonate, CaCO3) is one of the most common minerals and comes in nearly every color: colorless, white, yellow, orange, pink, blue, green, gray, and brown, depending on impurities. Luster is vitreous to resinous or pearly, and it ranges from transparent (Iceland spar) to opaque. Crystal habits are extremely varied — rhombohedrons, sharp scalenohedrons ("dogtooth"), prisms, nailhead forms, plus massive, granular, stalactitic, and banded (onyx-marble) forms. Cleavage rhombs that break into slanted parallelograms are a hallmark.
Step-by-Step Field-ID Checklist
- Apply a drop of dilute acid — calcite fizzes vigorously even on a solid surface (the single best test).
- Test hardness — Mohs 3; a steel knife or nail scratches it, but a copper coin barely does.
- Break or cleave it — calcite shows perfect rhombohedral cleavage in three directions (not 90 degrees), splitting into rhombs.
- Check for double refraction — a clear cleavage rhomb laid over a line shows a doubled image.
- Streak test — white.
- Note luster and crystal form — vitreous, with rhombohedral or dogtooth crystals.
Key Diagnostic Tests
- Mohs hardness: 3 — the standard reference; scratched by a knife, scratches a fingernail.
- Acid: vigorous effervescence in cold dilute HCl on a solid surface — diagnostic.
- Cleavage: perfect in 3 directions forming rhombohedra (cleavage angles ~75/105 degrees).
- Double refraction: strong; clear pieces double a printed line beneath them.
- Streak: white.
- Density: about 2.71 g/cm3.
- Not magnetic; not reactive to a magnet.
Common Look-Alikes and How to Tell Them Apart
- Quartz: much harder (Mohs 7, scratches glass), does not fizz in acid, and has no cleavage. Calcite is soft and effervesces.
- Dolomite: very similar carbonate, but dolomite fizzes only weakly, and mainly when powdered or in warm acid; calcite fizzes briskly on a solid surface.
- Aragonite: same composition but orthorhombic — lacks calcite's rhombohedral cleavage and is slightly harder/denser; often forms needles or coral-like masses.
- Gypsum: softer (Mohs 2, scratched by a fingernail) and does not fizz.
- Fluorite: harder (4), has octahedral cleavage, and does not react to acid.
- Barite: much heavier (density ~4.5) and does not fizz.
Where Calcite Is Typically Found
Calcite is the main mineral of limestone, marble, chalk, and travertine, and forms speleothems (stalactites/stalagmites) in caves. Superb crystals come from hydrothermal veins and vugs worldwide — famous localities include the Tri-State mining district (USA), Mexico, England (Egremont), and Iceland (optical Iceland spar).
Frequently asked questions
How can you tell if a stone is calcite?
Calcite fizzes vigorously when a drop of dilute acid (even vinegar slowly) touches it, is soft at Mohs 3, cleaves into rhombs in three directions, and clear pieces show double refraction. That combination is conclusive.
Calcite vs quartz — how do you tell them apart?
Quartz is hard (Mohs 7), scratches glass, has no cleavage, and does not react to acid. Calcite is soft (Mohs 3), cleaves into rhombs, and fizzes in acid.
Calcite vs dolomite — what's the difference?
Both are carbonates, but calcite fizzes briskly in cold dilute acid on a solid surface, while dolomite reacts only weakly and usually needs to be powdered or warmed before it fizzes noticeably.
What does calcite look like?
Calcite can be almost any color with a glassy to pearly luster, commonly forming rhombohedral or dogtooth crystals, or massive, banded, and stalactitic shapes.
Calcite identified by the community
Recent Calcite specimens identified with Rock Identifier.
