Calcite

What it actually is

Calcium carbonate, CaCO3, trigonal, hardness 3, with perfect cleavage in three directions producing rhombohedral fragments.

Calcite holds the record among common minerals for the number of distinct crystal habits described, with hundreds of forms recognised. It occurs as scalenohedra called dogtooth spar, as rhombohedra, as nailhead spar, as fibrous and massive material, and as the fine-grained calcite that makes up limestone and chalk.

Iceland spar, the optically clear variety, shows birefringence strongly: light entering it splits into two rays travelling at different speeds, so a line viewed through a cleaved rhomb appears doubled. That property was central to early optics, and Bartholin's 1669 description of it led eventually to the understanding of polarised light.

Calcite fluoresces in many colours depending on trace elements, and manganese-bearing material glows red under ultraviolet light.

It dissolves readily in dilute acid, which is the standard field test for it.

Where it comes from

Calcite is one of the most abundant minerals on earth and occurs almost everywhere sedimentary or metamorphic carbonate rock exists.

Iceland spar was quarried at Helgustadir in eastern Iceland, which supplied optical-grade material for scientific instruments for centuries; the site is now protected.

Mexico produces enormous quantities of orange and honey calcite, China supplies specimen and decorative material, and Britain's Pennine mining districts produced classic specimens alongside fluorite and galena.

Because it is so common, decorative calcite is inexpensive, and the market is dominated by a small number of high-volume sources.

Onyx marble, sold as Mexican onyx or Egyptian alabaster, is banded calcite and is a completely different material from the chalcedony called onyx.

What the traditions claimed

Calcite as such has very little traditional lore, because it was too common and too varied to be treated as a distinct precious material.

Iceland spar is the exception, and its claim is genuinely interesting. It is one of the three leading candidates for the solarsteinn, the Viking sun-stone described in Icelandic sources as a way of locating the sun through overcast sky. A calcite crystal was recovered from the wreck of an Elizabethan ship in the Channel, showing the technique was known and carried centuries later, and experiments have demonstrated that a skilled user can locate the sun to within a few degrees using birefringence and polarised skylight.

That is the only substantial historical claim attached to the mineral, and it is unproven for the Norse period specifically.

Modern practice reads calcite by colour, assigning different meanings to orange, green, blue and honey varieties, which is essentially the colour ray system applied to one mineral.

How it is worked with

Calcite is a specimen and display mineral. At hardness 3 with three cleavage directions and acid solubility, it will not survive wear or carrying.

A cleaved Iceland spar rhomb is genuinely worth owning, because the double refraction is one of the very few optical effects in mineralogy that anyone can see immediately with no equipment: put it on a printed line and the line doubles.

Coloured calcites are cheap, attractive and useful as shelf objects, and the colour ray readings apply to them as they would to any coloured stone.

Keep them dry and away from acid, dust with a soft brush, and do not put them in water.

For anything you want to carry, quartz is the same price and survives.

Buying it honestly

Calcite is too cheap to fake, but dyeing is very common. Bright blue, green, purple and pink calcites are usually dyed, since natural calcite runs to white, honey, orange, yellow, soft green and pale blue.

Dye pools along cleavage planes and in cracks under magnification.

The main misrepresentation is naming. Onyx marble sold as onyx is banded calcite, not chalcedony, and will scratch with a steel blade and fizz with vinegar where real onyx will not. Large cheap onyx bowls, lamps and vases are always calcite.

Some material sold as optical calcite is glass, and the double refraction test settles it instantly: real Iceland spar doubles a printed line, glass does not.

Aragonite, another calcium carbonate polymorph, is sometimes sold as calcite.