Calcite. Iceland spar. Helgustadir, Iceland.19th century.
There are very few specimens of Iceland spar of this size and quality, even from the world’s best known source, the Helgustadir mine in Iceland.
Iceland spar is a particularly clear variety of the common mineral calcium carbonate (calcite or calcspar). It is also referred to as Iceland spar after the Helgustadir mine in Iceland, the source of the best known and finest specimens. The mine was in operation from 1855 until 1925, and was declared a natural monument in 1975. In 1669 Erasmus Bartholin discovered the optical phenomenon of double refraction using a crystal from this mine, writing enthusiastically: “On the other hand, those who prefer a knowledge of unusual phenomena to amusements will, I hope, take no less pleasure in a new body, a transparent crystal that was recently brought to us from Iceland and which is perhaps one of the greatest wonders ever brought forth by Nature.”
One of Bartholin’s experiments is easy to reproduce: a cross scratched on the back of a piece of Iceland spar appears to be double when viewed through the crystal. This effect is due to the fact that, depending on its direction of propagation in the crystal, a ray of light is differently refracted and polarized. Many minerals exhibit the property of double refraction, though it is particularly pronounced in the case of Iceland spar.
Even in the 17th century, Iceland spar was no longer “new”, as Bartholin wrote. The Vikings valued it as a stone bringing peace and happiness, and it was apparently they who first brought it to the continental Europe in the 8th century.