Iolite is a blue gemmy form of cordierite. Cordierite is a cyclosilicate. It is usually found in rocks which have been altered by contact metamorphism and is commonly found in hornfels (a relatively high temperature zone near the contact aureole). It can be used as part of an assemblage of minerals to indicate the metamorphic history of a rock, but cannot be used on its own as it is very persistent.
Iolite displays strong pleochroism. When a translucent sample is held to the light it will be grey/yellow from some angles and dark blue from others.