Iolite
Cordierite group
Color: shades of blue to blue-purple
Moh's hardness: 7 - 7.5
The name iolite comes from the Greek "ios", which means violet. Iolite is usually a purplish blue when cut properly, the richer the blue the better. Iolite is a quite hard and durable stone with a hardness of 7 - 7.5 on the Moh's scale, and the abundant resources of this gem make it relatively affordable.
Pleochroism, which means "multicolouredness", is a phenomenon that is very prominent in iolites and is seen as three different color shades in the same stone: violet blue, yellow gray and a light blue.
Viking mariners used thin pieces of iolite as the world's first polarizing filter when they ventured far out into the Atlantic Ocean, away from any coastline that could help them determine position. By looking through an iolite lens, they could determine the exact position of the sun and navigate safely.
Pleochroism may have been helpful in navigation but it makes things difficult for a gem cutter. If iolite is not cut from exactly the right direction it will not show its optimal color.
Iolite is mainly mined in India, Sri Lanka, Madagascar, Myanmar (Burma), Mozambique, Zimbabwe, and Brazil.