Iceland Spar: The Rock That Discovered Optics

The Icelandic sagas of the 10th century record the details of Viking voyages. They describe a mysterious "sunstone", which Scandinavian seafarers used to locate the Sun in the sky and navigate on cloudy days.

Iceland Spar is a remarkable form of calcite, a crystalline mineral known for its double refraction phenomenon. When light passes through Iceland Spar, it splits into two separate rays, creating a captivating visual effect. This unique property earned Iceland Spar its nickname "Viking Crystal," as it is said that ancient Norse seafarers used these crystals as navigation aids, especially in overcast conditions when the sky's polarization was less discernible.

It occurs in large readily cleavable crystals, easily divisible into rhombs, and is remarkable for its birefringence. This means that the index of refraction of the crystal is different for light of different polarization. A ray of unpolarized light passing through the crystal divides into two rays of perpendicular polarization directed at different angles, called double refraction. So objects seen through the crystal appear doubled.
Natural polished "Iceland Spar" (or) Optical Calcite
Natural polished "Iceland Spar" (or) Optical Calcite

Geological Origins and Formation of Iceland Spar

The origins of Iceland Spar are deeply embedded in the Earth's geological history. This gemstone is primarily formed from calcite minerals that crystallize under specific temperature and pressure conditions. It can be found in various locations around the world, but the finest and most well-known deposits are found in Iceland, where the gemstone derives its name. Iceland Spar often forms as transparent rhombohedral crystals, showcasing its optical properties to their fullest extent.

Properties of Iceland Spar

Iceland spar is a transparent crystal with a refractive index of 1.658. This means that it bends light more than most other transparent materials. Iceland spar also has a high birefringence, which is the difference in refractive index between two perpendicular polarizations of light. This property causes objects seen through Iceland spar to appear doubled.


Iceland Spar: The Rock That Discovered Optics
Researchers say this crystal found at the Alderney shipwreck near the Channel Islands could prove fabled Viking sunstones really did exist.

How did Iceland Spar help the Vikings?

Researchers studied a piece of Iceland spar discovered aboard an Elizabethan ship that sunk in 1592. They found that moving the stone in and out of a person's field of vision causes them to see a distinctive double dot pattern that lines up with the direction of the hidden Sun.

Iceland Spar: The Rock That Discovered Optics
Screenshot from the TV show Viking

The polarization of sunlight in the Arctic can be detected, and the direction of the sun identified to within a few degrees in both cloudy and twilight conditions using the sunstone and the naked eye. The process involves moving the stone across the visual field to reveal a yellow entoptic pattern on the fovea of the eye, probably Haidinger's brush.

When light passes through calcite crystals, it is split into two rays. The asymmetry in the crystal's structure causes the paths of these two beams to be bent by different amounts, resulting in a double image.

Natural polished "Iceland Spar" (or) Optical Calcite Iceland Spar: The Rock That Discovered Optics

Uses of Iceland Spar

Iceland spar has a variety of uses, including:

  • Polarizing sunglasses: Iceland spar is used to make polarizing sunglasses because it can block out unwanted glare. This makes it easier to see in bright sunlight.
  • Optical instruments: Iceland spar is used in a variety of optical instruments, such as microscopes, telescopes, and goniometers. It is also used in laser systems and other devices that require polarized light.
  • Scientific research: Iceland spar is used in scientific research to study the properties of light. It is also used to measure the polarization of light from stars and other astronomical objects.
  • Metaphysical uses: Iceland spar is sometimes used for metaphysical purposes, such as healing and divination. It is believed to promote clarity of thought and intuition.

Caring for Iceland Spar

To maintain the pristine beauty of Iceland Spar, proper care is essential. Avoid exposing the crystal to direct sunlight for extended periods, as excessive UV exposure may alter its color. Gently clean Iceland Spar using a soft, damp cloth to remove dust and debris, and store it away from other harder gemstones to prevent scratching. 

Iceland Spar: The Rock That Discovered Optics/ Polished Optical Calcite Rhomb from Brazil
Polished Optical Calcite Rhomb from Brazil

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