IOLITE
Violet-blue iolite (also known as cordierite) has been called water sapphire because of its similarity to blue sapphire when cut. It can be recognized by its strong pleochroism *, visible without equipment, which gives the gem its other name of dichroite. The best blue colour is seen down the length of prismatic crystals; they may appear colourless when viewed across.
* pleochronic - term used to describe a gem that appears two or more different colours or shades when viewed from different directions.
Hardness: 7