Iceland Spar

Iceland Spar is a clear, transparent variety of calcite (calcium carbonate) renowned for its exceptionally strong double refraction (birefringence). Looking through a cleaved rhombohedron of Iceland spar, a single line or dot appears doubled, a dramatic demonstration of how the crystal splits light into two polarized rays.

The material is named for Iceland, where large, flawless masses were historically quarried, especially from the Helgustadir deposit. It played a key role in the history of optics and the study of polarized light.

Mineralogically it is ordinary calcite, crystallizing in the trigonal system with perfect rhombohedral cleavage, but of unusually high optical clarity.

Iceland spar forms as calcite crystallizing from calcium- and carbonate-rich solutions, typically in cavities, veins, and vugs within basaltic lava flows and other rocks. Slow growth in open spaces allows large, clear, well-ordered crystals to develop.

The classic Icelandic material occurred in vesicular basalt, where hydrothermal fluids deposited exceptionally clear calcite. Similar optical calcite is found in Mexico, the United States, and elsewhere in hydrothermal and cavity settings.

Because calcite is soluble and abundant, clear masses large and flawless enough for optical use are the exception, which made fine Iceland spar historically valuable.

The defining test is double refraction: place a clear cleavage rhomb over a line or dot and you will see two images. The stone is colorless and transparent, with a hardness of only 3 (easily scratched by a knife) and a vitreous to pearly luster.

Iceland spar breaks into perfect rhombohedra (slanted 'cubes') along three cleavage directions, and it fizzes vigorously in cold dilute hydrochloric acid, confirming carbonate. Streak is white.

Distinguish from clear quartz (much harder at 7, no easy cleavage, far weaker doubling) and from glass (no cleavage, no double refraction, no acid reaction). The strong double image plus easy cleavage and acid fizz are diagnostic.

Historically, Iceland spar was essential in optics, used to build Nicol prisms and other polarizing devices for microscopes and instruments before modern polarizing films. It was central to nineteenth-century studies of polarized light.

It is also famously proposed as the Viking 'sunstone' (sólarsteinn), a crystal possibly used to locate the sun through clouds by detecting polarized skylight, though this remains debated.

Today it is mainly a teaching specimen, collector's mineral, and curiosity. Metaphysically calcite is associated with clarity and amplification, a spiritual rather than scientific use. It is too soft and cleavable for durable jewelry.