Iceland occupies a singular position in the history of gemmology and optical science not because of rubies, sapphires, or any conventional gemstone, but because of a single extraordinary mineral: transparent, cleavage-perfect calcite known to science and trade as Iceland spar. The island nation — situated astride the Mid-Atlantic Ridge and formed almost entirely from volcanic basalt — is geologically inhospitable to most gem-bearing pegmatites or metamorphic terranes. Yet within its eastern highlands lies Helgustaðir, a deposit that supplied the world's finest optical-grade calcite for nearly three centuries and that remains the type locality against which all doubly refractive calcite is measured.

Geological Context

Iceland is a product of the Iceland hotspot combined with the divergent boundary of the North American and Eurasian tectonic plates. The island's bedrock is overwhelmingly basaltic, with extensive lava flows, hyaloclastite ridges, and geothermal fields. This volcanic character is almost entirely unfavourable for the formation of gem-quality corundum, beryl, chrysoberyl, or the silica-rich pegmatites that yield tourmaline and topaz. Hydrothermal activity does produce zeolites, calcite, and occasional quartz within vesicular basalt cavities — amygdaloidal fillings that are mineralogically interesting but of negligible commercial gem value.

The exception is the calcite veining associated with low-temperature hydrothermal systems. At Helgustaðir, in the Reyðarfjörður district of eastern Iceland, exceptionally pure calcium carbonate was deposited in large, open fractures within the basalt, producing rhombohedral cleavage masses of unusual size and transparency. Crystals and cleavage blocks from this locality reached dimensions of tens of centimetres, with a clarity and freedom from inclusions rarely matched elsewhere in the world.

Helgustaðir and the Discovery of Double Refraction

The Helgustaðir mine — sometimes rendered Helgustaðanámur in Icelandic — was known to local inhabitants well before it attracted scientific attention. The first documented European scientific description of the material dates to 1669, when the Danish physician and natural philosopher Erasmus Bartholin published his landmark treatise Experimenta Crystalli Islandici Disdiaclastici. Bartholin had received specimens of the Icelandic calcite and observed that objects viewed through it appeared doubled — a phenomenon he described with rigour and named double refraction, or birefringence. His work established the optical phenomenon as a subject of scientific enquiry and placed Iceland spar at the centre of early optics.

Christiaan Huygens subsequently used Iceland spar in his investigations into the wave theory of light, published in his Traité de la Lumière of 1690. Huygens recognised that the two refracted rays behaved differently and laid the conceptual groundwork for what would later be understood as polarisation. Isaac Newton also experimented with Iceland spar, though his corpuscular theory of light led him to different interpretations. In the nineteenth century, Augustin-Jean Fresnel and others used the material to demonstrate and study linear polarisation definitively. Iceland spar was thus not merely a mineralogical curiosity but an essential laboratory instrument in the development of physical optics.

Industrial and Scientific Use

By the nineteenth century, demand for optical-grade Iceland spar had become substantial. The primary application was in the construction of Nicol prisms — optical devices fabricated by cementing two rhombohedra of calcite together with Canada balsam after one had been cut and repositioned. Nicol prisms were the standard polarising element in polarimeters, petrographic microscopes, and saccharimeters throughout the Victorian era and into the early twentieth century. Every geological thin-section microscope of that period relied on calcite from Helgustaðir or equivalent localities.

The Helgustaðir deposit was actively quarried from the seventeenth century onward, with peak extraction occurring in the nineteenth and early twentieth centuries. By the mid-twentieth century, the most accessible large, flawless material had been substantially exhausted, and synthetic polarising films — particularly the Polaroid sheet developed by Edwin Land in the 1930s — had rendered natural calcite prisms largely obsolete for most applications. The mine ceased commercial operation, and the site is today protected as a natural monument by the Icelandic government.

Iceland Spar as a Gemmological Reference Material

Within gemmology, Iceland spar retains importance as the canonical demonstration specimen for birefringence. Calcite has a refractive index of approximately 1.486 for the ordinary ray and 1.658 for the extraordinary ray — a birefringence of 0.172, among the highest of any common mineral. This extreme double refraction means that a single calcite rhombohedron placed over printed text will produce two clearly separated images visible to the naked eye, making it the most immediately legible demonstration of the phenomenon available to students and educators.

Gemmological training programmes worldwide use Iceland spar cleavage rhombs precisely because the effect is unambiguous and requires no instrumentation. The material also serves as a calibration reference for refractometers and polariscopes in some laboratory contexts, though modern instruments are calibrated against more stable standards.

It is worth noting that calcite itself is not a gemstone in any commercial sense — its perfect cleavage in three directions, hardness of only 3 on the Mohs scale, and sensitivity to acids render it entirely unsuitable for jewellery. Faceted calcite exists as a collector's curiosity, and some translucent to opaque calcite varieties — including alabaster, onyx marble, and travertine — have been used decoratively for millennia. However, these are distinct from the optical-grade transparent material associated with Iceland.

Viking Navigation and the "Solar Stone" Hypothesis

A persistent and scientifically examined hypothesis proposes that Norse navigators used Iceland spar as a navigational aid — a so-called sólarsteinn, or solar stone — to locate the sun's position in overcast conditions by detecting the polarisation pattern of skylight. The hypothesis is grounded in the optical reality that calcite can indeed be used in this manner: when rotated while viewing skylight through a cleavage rhomb, the relative brightness of the two images changes in a manner that allows the sun's azimuth to be estimated even when the sun itself is below the horizon or obscured by cloud.

The hypothesis gained renewed attention following the 2013 discovery of a calcite crystal aboard the wreck of an Elizabethan-era English warship, the Alderney, lost in 1592. Laboratory analysis confirmed the specimen was Iceland spar and that it was found in proximity to navigational instruments, lending circumstantial support to the idea that such crystals were used at sea. Whether Norse navigators specifically employed Iceland spar remains unproven by direct archaeological evidence, but the optical principle is sound and the hypothesis is taken seriously in both archaeological and scientific literature.

Iceland Today: Gemstones and Minerals

Iceland produces no gemstones of commercial significance. Collectors may find zeolite minerals — stilbite, chabazite, thomsonite, and related species — in basalt cavities, and these occasionally display attractive crystal habits. Obsidian, the volcanic glass formed from rapidly cooled rhyolitic lava, occurs in Iceland and has been used as a cutting material, though Icelandic obsidian is not distinguished in the gem trade. Quartz, including amethyst, has been reported from hydrothermal veins but not in quantities or qualities that have attracted commercial interest.

The Helgustaðir site itself is accessible to visitors and is recognised for its historical significance. The Icelandic Institute of Natural History maintains records of the locality, and specimens from the deposit are held in major natural history museum collections worldwide, including the Natural History Museum in London and the Smithsonian Institution.

Summary

Iceland's contribution to gemmology and science is concentrated entirely in a single mineral from a single locality. Helgustaðir calcite — Iceland spar — shaped the early understanding of light, enabled a century of polarising microscopy, and remains the reference specimen for birefringence in gemmological education. As a gem-producing nation, Iceland is negligible; as the type locality for one of science's most consequential mineral specimens, it is irreplaceable.

Further Reading

• Gems & Gemology — GIA Publications Archive
• International Gem Society: Calcite Gemstone Information