carbonates
Calcite
Ca(CO3)
- Also known as
- Iceland sparoptical calciteonyx marbledogtooth spar
- Crystal system
- trigonal
- Hardness
- 3 on the Mohs scale
- Lustre
- vitreous
- Colour
- Colourless, white, or almost any colour with impurities
- Where it forms
- Everywhere water meets calcium. It is the mineral of limestone, marble, chalk, stalactites, shells, coral skeletons and the scale in a kettle.
- Chiefly from
- worldwideIcelandElmwood, Tennessee, United StatesMexico
Why it is that colour
Pure calcite is colourless. Manganese gives pink, iron gives yellow and brown, and cobalt gives an intense pink. It also fluoresces in a wide range of colours.
Safety
Safe to handle. It dissolves readily in weak acid — a drop of vinegar will fizz — which is the standard field test and also the reason calcite ornaments should be kept away from cleaning products.
Where it comes from
Among the most abundant minerals on earth and quarried in vast quantity as limestone and marble. Specimen material is a by-product.
Historical use
Defines position 3 on the Mohs scale. Limestone and marble are calcite, so most monumental architecture before concrete is made of it, and so is the lime in mortar and cement. Biologically it is the material of shells, and the chalk cliffs of southern England are the accumulated plates of microscopic algae.
Recorded as history and attributed where possible. That a stone was used for something establishes that people used it, not that it worked.
What is attributed to it, and since when
Calcite as such carries no significant traditional symbolism, though the rocks made of it — marble, alabaster, limestone — carry a great deal. Iceland spar has a genuine and remarkable technical history, described below. Colour-coded meanings for calcite are entirely modern.
Described, not endorsed. Dating a claim is not the same as assessing it — but an association that first appears in a book published in 1985 is not an ancient inheritance, whatever the label says. A sincere account of feeling better is compatible with the stone doing nothing — see the placebo effect.
Worth knowing
Iceland spar deserves to be much better known. It is transparent calcite, and it exhibits strong double refraction: look through it and you see two images, because it splits light into two rays polarised at right angles to each other. Rasmus Bartholin described the effect in 1669, and it was central to working out that light is a wave — Huygens, Newton, Fresnel and Malus all used it.
It is also the best candidate for the Viking sunstone. A crystal of it can locate the sun through thick overcast, because the polarisation pattern of the sky points back at the sun even when the sun is invisible. One was recovered from an Elizabethan shipwreck off Alderney in 2013, aboard a vessel that also carried compasses — which suggests it was there as a backup for when the compass was disturbed by the ship's own iron.
Identity checked against IMA-CNMNC master list, June 2025.