Iolite or cordierite, also known as the "water sapphire" when gem quality, is a unique gemstone which demonstrates pleochroism or changes color depending on the direction your hold the crystal. Iolite will change form blue to grey when the crystal is clean and clear. 

Legend has it iolite is called the Viking Compass Stone and is said that thin slices of iolite served as glare-reducers and polarizing filters helping ancient Viking navigators locate the sun on cloudy days. This allowed the Nordic mariners to pinpoint their own location on the seas.

Iolite is a good stone for jewelry as a a magnesium iron aluminum cyclosilicate in the silicate family and it is fairly hard at a 7-7.5 on the mohs scale.