Iolite is strongly pleochroic, which means that it shows different colors according to its orientation: blue-violet, colorless, yellow-brown, or light blue, "like spring water".
This property brought the name "water sapphire", today prohibited.
It was used by the Vikings because of this optical feature. These were the first polarization filter compasses in the world. A lens made of iolite, made it possible to determine the exact position, according to the sun.
This polarization property of the rope allowed the use of this «solar compass». Its efficiency was ideal when the sun was close to the horizon. Therefore, its use was optimal under high latitudes. The Vikings were able to sail to the new world and return...
The pleochroism of the Iolite, which was useful to the Viking navigators, makes the size difficult to the lapidary. If the stone is not cut exactly in the right direction, whatever the shape of the rough, the color will not appear at best. The gemstones show colors from deep blue to violet blue, light blue or blue gray.
For more information on the mineralogy of Iolite (Cordiérite), see Mindat.org->
