Vikings
It is believed by some historians that Iolite was the gemstone used by ancient Vikings, to help them navigate the oceans. Iolite has a particular optical property called Pleochroism, which changes the color of the stone depending on which angle light enters it. What made it especially useful to the Vikings was Iolite's sensitivity, it picks up light from the sun even on cloudy days.

On cloudy days where the Vikings couldn't use the sun for navigation, an iolite crystal was set in a frame of wood close to the ship's navigator, as soon as the boat veered off-course light would enter the stone at a different angle, displaying a different hue.

A tiny crystal at the base of Viking dominance
The success of the Vikings was attributed to their technologically superior ships and seafaring skills. Using their longships, they would travel vast distances until they found a coastal town where they had a tactical advantage, after which they would strike swiftly and brutally. The effectiveness of this tactic gave the Vikings their reputation as fearsome raiders and pirates, allowing them to continue raiding Europe for over two hundred years.

Recent discoveries convinced some archaeologists that aboard each Viking ship was a little iolite crystal, which enabled them to out-navigate every other nation at that time.