From Helene Campbell
Iolite is often mistaken for Sapphire, even though it is softer, in reality it is rarer stone than Sapphire.
Iolite is often referred to as "Water Sapphire" or Dichroite. Iolite's name comes from the Greek ios, meaning violet. It can show two colours in the stone, this is technically called Pleochroism.
Fun fact: Iolite was called "Vikings' Compass" because of its usefulness in determining the direction of the sun on overcast days, the Vikings having used it for this purpose.