Iceland spar is a variety of calcite (CaCO₃) distinguished by exceptional optical clarity and one of the strongest birefringences of any naturally occurring mineral. A colourless, transparent rhombohedral crystal, it splits any beam of light passing through it into two separate, perpendicularly polarised rays — a phenomenon so pronounced that a single crystal placed over printed text renders every letter as a distinct double image. First described from large, gem-quality cleavage masses found in eastern Iceland in the seventeenth century, the material became the foundational substance of experimental optics and remains, to this day, the standard reference example of birefringence in gemmological education. Though Iceland is no longer a commercial source, the name persists as a technical term for high-quality optical calcite regardless of provenance.

Mineralogy and Crystal Chemistry

Calcite belongs to the trigonal crystal system, space group R3̄c, and is the most stable polymorph of calcium carbonate under surface conditions. Its structure consists of alternating layers of calcium ions and planar carbonate groups (CO₃²⁻), the latter tilted relative to the crystallographic c-axis. This arrangement produces the mineral's characteristic rhombohedral cleavage — perfect in three directions intersecting at approximately 74° and 106° — which allows large, optically homogeneous cleavage rhombs to be produced simply by cleaving a crystal mass along its natural planes. Iceland spar represents calcite at its purest: the near-total absence of iron, manganese, or other chromophores accounts for its water-clear transparency, while the structural perfection of large single crystals minimises scattering.

The refractive indices of calcite are nω = 1.658 and nε = 1.486, yielding a birefringence of 0.172 — among the highest of any common mineral and roughly seventeen times greater than that of quartz. This extreme difference in refractive index between the ordinary and extraordinary rays is the direct consequence of the anisotropic polarisability of the carbonate ion: light vibrating parallel to the carbonate planes experiences a very different electronic environment from light vibrating perpendicular to them. The material is uniaxial negative, meaning the extraordinary ray travels faster than the ordinary ray when propagating perpendicular to the optic axis.

The Icelandic Discovery and Early Scientific History

The deposit that gave Iceland spar its name is the Helgustaðir mine (also written Helgustaðanáma), located near Reyðarfjörður in eastern Iceland. The site was apparently known to local inhabitants well before formal scientific documentation, but it entered European scientific literature following Erasmus Bartholin's 1669 treatise Experimenta Crystalli Islandici Disdiaclastici, in which he described the phenomenon of double refraction with systematic precision. Bartholin's observations — that a single image seen through the crystal became two, and that rotating the crystal caused the images to behave differently — laid the empirical groundwork for subsequent theoretical work by Christiaan Huygens, whose Traité de la Lumière (1690) offered the first wave-mechanical explanation of double refraction using Iceland spar as the primary experimental subject.

The Helgustaðir deposit was remarkable not merely for the quality of its crystals but for their size. Individual cleavage masses weighing several kilograms were documented, and the mine supplied European scientific institutions for more than two centuries. By the mid-nineteenth century, however, intensive extraction had largely exhausted the most accessible material, and the mine was formally abandoned. The site is today protected as a natural monument under Icelandic heritage legislation.

The Viking Sunstone Hypothesis

A persistent and scientifically debated question concerns whether Norse navigators used Iceland spar as a sólarsteinn — a sunstone — to locate the sun's position in overcast or twilight conditions before the magnetic compass reached northern Europe. The hypothesis, first proposed in the 1960s by Danish archaeologist Søren Thorkild Ramskou, rests on the fact that a calcite rhomb, held up and rotated while observing the sky, can detect the plane of polarisation of scattered skylight, which is geometrically related to the sun's azimuth even when the sun itself is below the horizon or obscured by cloud.

The argument gained renewed attention in 2013 when a calcite crystal was identified among the navigational instruments recovered from the wreck of an Elizabethan warship, Alderney, lost in 1592. Laboratory testing confirmed that the crystal's optical properties were consistent with use as a polarising navigational aid. While no Iceland spar artefact has been definitively identified from a Viking-period archaeological context, experimental studies published in peer-reviewed optics journals have demonstrated that the method is theoretically viable to within a few degrees of accuracy under realistic sky conditions. The question remains open in the archaeological literature, though the optical plausibility of the sunstone hypothesis is no longer seriously disputed.

Optical Applications and Scientific Importance

The practical exploitation of Iceland spar's birefringence accelerated dramatically in the nineteenth century with the development of polarising optical instruments. The Nicol prism, invented by William Nicol in 1828, was constructed by cementing two calcite rhombs together with Canada balsam at a calculated angle such that the ordinary ray undergoes total internal reflection at the interface and is eliminated, while the extraordinary ray passes through as a single, linearly polarised beam. Nicol prisms became the standard polarising element in petrographic microscopes, saccharimeters, and spectrometers for nearly a century, and the demand they created drove commercial mining of optical calcite from Iceland and, subsequently, from Mexico, the United States, and southern Africa.

Later designs — including the Glan-Thompson prism, the Glan-Taylor prism, and the Wollaston prism — refined the geometry and improved transmission efficiency, but all relied on the same fundamental property of calcite. In the twentieth century, synthetic calcite and alternative birefringent materials such as yttrium orthovanadate and beta-barium borate have largely displaced natural Iceland spar in high-precision optical applications, but calcite remains in use in educational polariscopes and demonstration instruments precisely because its birefringence is so large that the double-image effect is immediately visible to the naked eye.

In gemmological practice, the polariscope — an instrument consisting of two polarising filters oriented at 90° to one another — is used routinely to distinguish singly refractive from doubly refractive stones, to identify anomalous double refraction in glass and synthetic materials, and to detect strain in gems. The conceptual and instrumental lineage of this tool runs directly back to Iceland spar and the Nicol prism.

Physical and Optical Properties

• Chemical formula: CaCO₃ (calcium carbonate)
• Crystal system: Trigonal (rhombohedral)
• Cleavage: Perfect rhombohedral in three directions; cleavage angle approximately 74° / 106°
• Hardness: 3 on the Mohs scale
• Specific gravity: 2.71 (essentially constant for pure calcite)
• Refractive indices: nω 1.658, nε 1.486
• Birefringence: 0.172 (extreme; uniaxial negative)
• Lustre: Vitreous
• Transparency: Transparent to translucent; gem-quality material is water-clear
• Fluorescence: Variable; may fluoresce pink or orange under longwave ultraviolet, depending on trace manganese content
• Solubility: Readily attacked by dilute acids, including perspiration; unsuitable for jewellery subject to wear

Sources and Current Availability

Although Iceland is no longer a commercial source, optical-grade calcite is mined in several regions. Mexico has historically produced large, clear rhombs, particularly from the state of Chihuahua. Substantial deposits occur in the United States (notably in Missouri, New Mexico, and South Dakota), in Namibia, and in the Hunan and Guizhou provinces of China. Brazilian material, often associated with carbonate-hosted mineral deposits in Minas Gerais, is also encountered in the trade. The quality criterion for optical calcite is straightforward: freedom from inclusions, cleavage fractures, and colour zoning over a usable aperture. Crystals meeting this standard are sold primarily to educational suppliers, optical instrument manufacturers, and collectors; gem-quality faceted calcite exists but is essentially a collector's curiosity given the mineral's low hardness and perfect cleavage.

In the mineral specimen market, Iceland spar commands consistent interest from collectors who value the visual drama of the double-image effect. Large, perfectly clear rhombs from any locality are desirable; Helgustaðir material, when provenance can be documented, carries additional historical significance and commands a premium accordingly.

Gemmological Significance

Within gemmology, Iceland spar occupies a dual role: as a subject of study in its own right and as the conceptual anchor for understanding birefringence across all gem species. The GIA's gemmological curriculum uses calcite as the canonical example of extreme birefringence, contrasting it with the moderate birefringence of zircon (0.059), peridot (0.036), and tourmaline (up to 0.035), and the near-zero birefringence of spinel and garnet. Understanding why calcite's birefringence is so much larger than that of silicate gems — a consequence of the highly anisotropic carbonate ion rather than the more isotropic silicate tetrahedron — provides a conceptual foundation for interpreting refractive index measurements across the gem kingdom.

Calcite also appears as an inclusion mineral in a number of gem hosts. Calcite inclusions in ruby and sapphire from marble-hosted deposits (notably Mogok, Mong Hsu, and the Luc Yen district of Vietnam) are gemmologically significant because their presence, combined with the absence of iron-related fluorescence quenching, contributes to the intense red fluorescence and vivid colour saturation characteristic of the finest Burmese and Vietnamese rubies. In such contexts, calcite is not merely a contaminant but a petrological indicator of the host-rock environment.

Treatments and Simulants

Iceland spar and optical calcite generally are not treated in any meaningful gemmological sense. The material's value lies entirely in its natural optical perfection; any treatment that introduced stress or altered the crystal structure would degrade rather than enhance its utility. Coloured calcite varieties (honey calcite, cobaltoan calcite, and the like) are sometimes encountered in the decorative stone trade, but these are distinct from the optical-grade Iceland spar discussed here.

As a simulant, calcite is occasionally encountered masquerading as white topaz or colourless quartz in low-value commercial goods, though its extreme birefringence, low hardness, and perfect cleavage make identification straightforward under even basic gemmological testing. The double-image effect visible through a loupe is, in itself, a near-definitive diagnostic: no common colourless gem simulant approaches calcite's birefringence of 0.172.

Conservation and Heritage Status

The Helgustaðir mine site near Reyðarfjörður is listed as a protected natural monument in Iceland, and the removal of material from the site is prohibited. The mine workings, though modest in scale by industrial standards, represent a site of exceptional importance in the history of science: it was here that the empirical observations leading to the wave theory of light, the development of polarimetry, and ultimately the electromagnetic theory of light were grounded. Proposals have been made periodically to develop the site as a heritage destination, and it is accessible to visitors, though the most productive excavations were exhausted long ago.

Further Reading

• GIA Gem Encyclopedia: Calcite
• International Gem Society: Calcite
• Gems & Gemology: Birefringence in Gems (GIA)