Single calcite crystals display an optical property called birefringence (double refraction). This strong birefringence causes objects viewed through a clear piece of calcite to appear doubled. (The lines are single and when you rotate the crystal 90 degrees, they will appear doubled. When you rotate another 90 degrees, they will appear single.) The birefringent effect (using calcite) was first described by the Danish scientist Rasmus Bartholin in 1669.
Long before the invention of a compass, “the Vikings used a transparent calcite crystal -- also known as Iceland spar -- to fix the true bearing of the Sun, to within a single degree of accuracy.” Read more at: http://phys.org/news/2011-11-viking-sunstone-myth.html#jCp
When the United States entered World War II, high-grade optical calcite was used for gun sights, specifically in bomb sights and anti-aircraft weaponry. Iceland spar was considered a strategic resource by the U. S. government, which had classified studies done assessing the Iceland spar mines of South and Central American nations.
New sources were found first in South Africa and later in Mexico, the U. S., and South America. Even with these new sources, the supply was always limited. Worldwide mining produced a few tons of optical grade Iceland spar, at most, each year. Most of the optical calcite crystals used today are mined just like they were in the old days, which is why Iceland spar crystals remain rather pricey.
Optical calcite polarizers are used today as polarizers and beam-splitters in the laser and fiberoptics industries. The most common current use of calcite crystals is in Glan-type prisms;these can withstand very high luminance and are used to polarize laser light.
Metaphysically, Iceland Spar helps remove energetic blockages and assists in bringing clarity and understanding of the lessons behind current life situations. Like clear quartz it can be programmed to amplify the effect of one's intent. It can help one forgive oneself and others for past mistakes.
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