Optical Calcite (Iceland Spar): Properties, Double Refraction, Care

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What Is Optical Calcite?

Optical calcite is calcite (calcium carbonate, CaCO3) that is transparent and colourless, of a quality good enough to see clearly through. It is most often sold in the form of rhombohedra: blocks with flat faces, obtained by breaking a crystal along its cleavage planes (see cleavage and fracture). It is also called Iceland spar, after its main historical deposit.

Its remarkable property is double refraction: a light ray entering the crystal splits into two rays that leave at different points. If you place the rhombohedron over a line of text or a mark, you see two images. This separation is exceptionally strong: the two refractive indices are 1.658 and 1.486, a difference of about 0.17. For comparison, it is about 0.009 in quartz, and zero in glass or diamond.

Its physical properties are those of calcite: hardness 3, density 2.71, reaction to acids. For the basics, read the page on white calcite.

A Stone in the History of Physics

In 1669 the Danish physician Rasmus Bartholin described the double refraction of Iceland spar. Christiaan Huygens offered an explanation based on the wave theory of light in his Treatise on Light (1690). In 1808, Étienne-Louis Malus discovered the polarisation of light while observing through a crystal of this kind. In the 19th century, the Nicol prism (1828), cut from calcite, was used in polarised-light microscopes.

It is thus a mineral that helped people understand that light is a wave that can be polarised. Today, optical calcite is still used in precision optics, but it is often replaced by other synthetic crystals.

The deposit at Helgustadir, in Iceland, supplied most of the Iceland spar for centuries; it is now a nature reserve where extraction is prohibited. Today's pieces come mainly from other countries, such as Mexico or China.

The Viking "Sunstone": A Hypothesis

Icelandic texts from the 13th century mention a "sunstone" (sólarsteinn) that would allow the sun to be located on overcast days. In 1967, the Danish archaeologist Thorkild Ramskou proposed that it was a polarising crystal, like Iceland spar, used by Viking navigators. A piece found in the wreck of a 16th-century ship near Alderney (Channel Islands) revived the debate. In 2011, a team showed that such a crystal could indicate the direction of the sun to within a few degrees at twilight, but other tests have not reproduced this result.

It is therefore a hypothesis, neither proven nor disproven. The texts are, moreover, allegorical. We cannot claim that the Vikings used this crystal to navigate.

Traditional Symbolism

A pure, transparent crystal that "sees double", optical calcite is associated in modern crystal literature with clarity of mind, vision and the amplification of intentions. Authors readily link it to the third eye and the crown chakra.

These associations are modern conventions. They rest on the image of the clear crystal, not on ancient tradition or any demonstrated effect. Its greatest interest is visible to the eye: the doubling of images is a real phenomenon that you can show to a child.

Domain	Cited correspondence
Chakra	third eye or crown
Element	air or water, depending on the author
Keywords	clarity, discernment, vision

Try the Double Image Experiment

1. Draw a dot or a fine line in pencil on a white sheet of paper.
2. Place the rhombohedron over it, one face flat: you see two dots or two lines.
3. Slowly turn the stone over the sheet: one of the images stays fixed while the other turns around it.
4. Look through a polarising filter (polarised sunglasses lens): one of the images disappears at a certain angle.

This is also a very good identification test, reliable and safe. It shows that the crystal is birefringent, but not that it is calcite: other minerals double images too, though more weakly.

Care and Cleansing

• Scratches: hardness 3. Store it wrapped in fabric, apart from other stones, otherwise its faces will scratch and the crystal will lose its clarity.
• Acids: strictly to be avoided (vinegar, lemon, descalers).
• Cleaning: a soft or microfibre cloth, slightly damp, lukewarm water, careful drying. No soaking.
• Knocks and heat: the stone splits along its cleavage planes; avoid thermal shocks and prolonged intense sun.
• Symbolic cleansing: smoke, sound or moonlight rather than water (guide).

Recognising Genuine Optical Calcite

Sign	Optical calcite	Imitation or confusion
Image seen through the stone	doubled, with one image that turns	single image: glass, rock crystal (quartz doubles very little, and its shape is different)
Shape	rhombohedron with flat faces, angles of about 75° and 105°	cut cube or pyramid: a different material
Hardness	3: scratches very easily (no need to test it)	7: quartz; 5 to 6: glass
Interior	sometimes iridescent fracture planes	round bubbles: glass

Natural optical calcite may show cracks and inclusions: these are not disqualifying flaws. A perfect geometric shape and a total absence of inclusions should prompt questions. See real or fake stone and our commitments.

How to Choose

Choose a rhombohedron that is as clear as possible, with as few internal cracks as possible and no chips on the edges. The bigger and clearer it is, the more it costs; small pieces are affordable. For observation, a piece 3 to 5 centimetres across shows the double image very well.

Pairings: with rock crystal to compare two clear crystals, or with white calcite to see the same material in two appearances. To care for all your stones, read the guide.

Frequently asked questions

Also read

The meanings mentioned belong to tradition and symbolism. They are neither promises of results nor medical advice. Back to Guide to stones.