Iolite is called the violet stone. Its colour ranges include blue, brown, yellow, dark violet, grey and green. But gem quality iolite is generally bluish violet colour. This gemstone also exhibits pleochroic qualities. Which means that the violet blue colour is visible, until the stone is turned through 90 degrees. Then it will display a more bluish brown colour. And then if that stone is turned on its side, it may display a completely new shade of blue or brown.
Iolite was also called the 'Vikings Compass'. Because the pleochroic effect works by determining the direction that light waves are traveling. Very useful for determining the suns position on overcast days. With a well polished piece of iolite and a good understanding of the way that the colours shift within stone, in relation to the sun, and you could theoretically navigate your way across the Atlantic. Regardless of the weather, as it's the polarization that affects the stone. Not the sun. What an amazing reason for a gemstone based prized possession. [How to Catch a Robot Rat by Agnes Guillot, Jean-Arcady Meyer, Susan Emanuel (2010)]
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