Iceland spar: The Helgustadir calcite locality and its influence on the development of science

A site at Helgustadir, East Iceland, supplied large quantities of transparent cleavage rhombs of calcite (commonly called Iceland spar) from 1668 to 1925. These crystals played a significant role in the early development of several fields in the physical sciences including wave optics, crystallography, and crystal physics. From the mid-19th century onwards, most of the Nicol prisms which were used in petrographic microscopes, polarimeters and various other optical instruments were made of Iceland spar from the Helgustadir locality. These instruments contributed to advances in mineralogy and petrology, as well as in branches of physics, chemistry, biological sciences, and technology where polarized light was employed.