Iceland spar and its legacy in science

Abstract. In the late 17th century, Rasmus Bartholin and Christiaan Huygens investigated a curious optical property of crystals found at Helgustaðir in Eastern Iceland. This property which has been called double refraction, revealed in the 19th century a new aspect of light which turned out to be very useful as a probe of the internal structure of matter. Clear specimens of these crystals, an unusually pure variety of calcite, have since around 1780 been known as ''Iceland spar''. Few if any other localities yielding calcite crystals of comparable size and quality were discovered before 1900, and no alternatives for use in precision optical instrumentation were developed until the 1930s. Hundreds of tons of calcite were exported from Helgustaðir, mostly between 1850 and 1925. However, little information has been found on trading routes for the material of optical quality, so that some enigmas remain regarding its supply-demand situation. A study of the scientific literature in the period up to 1930 has revealed that results obtained with the aid of Iceland spar accelerated progress within the earth sciences (in mineralogy and petrology), physics, chemistry, and biology, even by decades. This has also influenced the development of technology and of medicine in various direct and indirect ways.