Gem library
Calcite
The carbonate of a thousand forms, king of birefringence
Calcite has the most varied crystal forms in the mineral kingdom: more than 800 described. It is also the absolute champion of birefringence — a transparent crystal placed over a text doubles it with spectacular clarity.
- Hardness (Mohs)
- 3
- Density
- 2.69 – 2.72
- Crystal system
- Trigonal
- Refractive index
- 1.486 – 1.658
- Cleavage
- Perfect on {1011}
Mineralogical identity
- Family
- Carbonates
- Species
- Calcite
- Variety
- Iceland spar, optical calcite, onyx marble
- Chemical formula
- CaCO₃
- Crystal system
- Trigonal (rhombohedral)
- Classification
- Collector's mineral
- Rarity
- Very common
- Indicative price
- €0.10 – 5/ct
Etymology
From the Latin calx, «lime»: heated, calcite decomposes into quicklime, the basis of every mortar since antiquity.
Physical & optical properties
- Hardness (Mohs)
- 3
- Density
- 2.69 – 2.72
- Refractive index
- 1.486 – 1.658
- Birefringence
- 0.172 (extreme)
- Cleavage
- Perfect on {1011} — three rhombohedral directions
- Fracture
- Conchoidal (rare)
- Lustre
- Vitreous to pearly
- Diaphaneity
- Transparent to opaque
- UV fluorescence
- Strong
- Pleochroism
- None
- Colours
- white, colourless, yellow, orange, pink, blue, green
Formation, geology & history
Iceland spar and double refraction
A crystal of transparent calcite splits a light ray into two rays polarised at right angles, which travel through the crystal at different speeds. The gap between the two indices, 0.172, is the highest among common minerals: through a plate of Iceland spar, every line appears clearly doubled. This discovery, made by Rasmus Bartholin in 1669 on Icelandic crystals, is the origin of all polarisation optics. The Nicol prism, cut from Iceland spar, was for a century the indispensable tool of polarising microscopy — and of the first measurements of polarised light.
An omnipresent mineral
Calcite makes up limestones, marbles, chalk, stalactites, shells, coral reefs and the skeletons of countless organisms. By volume it is one of the most abundant minerals in the Earth's crust and the main reservoir of mineral carbon on the planet. It is also the pillar of human construction: lime, cement, mortar and lime plaster all derive from its thermal decomposition. Its biological formation — biomineralisation — remains one of the liveliest research topics in mineralogy.
Collector varieties
The vocabulary is vast: Iceland spar (transparent optical), Mexican orange calcite, Madagascan blue calcite, Peruvian pink calcite, dogtooth (pointed scalenohedra), nailhead (flattened rhombohedra), phantom calcite, optical calcite. «Mexican onyx», the «alabaster» of many decorative objects and «onyx marble» are all banded calcite. Many calcites fluoresce intensely under UV in red-orange, making them spectacular pieces in a lamp-lit display case.
How to recognise it
- Effervescence in acidA drop of white vinegar is enough: calcite fizzes immediately, releasing CO₂. A decisive test never reproduced by quartz or glass.
- Scratched by a coinHardness 3: a copper coin marks it, a fingernail almost does. No hard stone behaves like this.
- Doubling of a lineOn transparent crystals, extreme birefringence visibly doubles lines seen through the stone.
Imitations, treatments & confusions
- Sold as citrine, precious aragonite or onyxOrange and honey calcites are often presented as nobler stones. The vinegar test and the scratch test unmask them instantly.
- Glass and resinNo effervescence, no rhombohedral cleavage, higher hardness. Resins give off a plastic smell on a hot point.
- Dyed calciteFrequent in cheap beads and pebbles, often sold under fanciful names. The dye concentrates in microfractures.
Deposits & origins
Everywhere in the world — Mexico (orange and honey calcites), Iceland (historic Iceland spar), United States (Tennessee, Missouri), China, Morocco, Romania, England (Cumbria), France (countless deposits)
Care & precautions
- Never prolonged water, never acid: calcite dissolves, including in slightly acidic water and lemon juice.
- A soft dry or barely damp cloth, dried at once. No household products.
- Very fragile: hardness 3 and triple perfect cleavage. It is scratched by contact with other stones and chips at the slightest knock.
- Ultrasonic cleaning is absolutely forbidden. Keep coloured calcites out of the sun, as they fade.
Traditions & crystal healing
- Chakras
- Varies with colour
- Zodiac signs
- Cancer · Leo
- Element
- Earth · Water
Tradition associates it with cleansing and clarity, each colour having its own domain: orange for momentum, blue for speech, green for calm, yellow for confidence. Its fragility makes it a stone you place in a room rather than wear, often as a large rough piece.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
How do I identify calcite for certain?
A drop of vinegar: immediate effervescence. Combine with the hardness test (scratched by a coin) and you have a certain identification.
Calcite or quartz?
Everything separates them: hardness 3 against 7, effervescence against inertness in acid, triple perfect cleavage against no cleavage, extreme birefringence against weak.
Can it go in water?
No. Calcite is soluble in slightly acidic water and its polish dulls quickly. A dry cloth is the only recommended care.