Iolite

Iolite is a blue silicate mineral that occurs as crystals or grains in igneous rocks. Iolite has a pleasing color of blue, but also a rather extraordinary optical property. The gemstone changes colors depending upon which angle it is viewed from. When the legendary Viking mariners sailed the wide ocean, they used thin pieces of iolite as the world's first polarizing filter. Looking through an iolite lens, they could determine the exact position of the sun, and navigate safely.