Iolite that is gem quality is sometimes used as a reasonably priced substitute for Sapphire and Tanzanite. Iolite has extreme pleochroism and is often a trichoic stone that shows three hues when viewed at different angles, violet, blue and colourless. Iolite colours vary from a bright sapphire blue to blue violet to yellowish gray to light blue as the light direction changes. Iolite was likely the world's first polarizing filter; Viking mariners used thin pieces of Iolite to navigate during their travels.
