Illerium was a precious substance frequently smuggled to planets where it was in demand.
Illerium possessed a high degree of instability, particularly upon contact with the atmosphere. This characteristic rendered it an essential component in the production of starship ejection seats and escape pods, providing the required thrust to launch the vehicle clear of the main vessel. Furthermore, it had a similar application to detonite, although with significantly reduced explosive power.
In the course of the Galactic War, a radioactive version, known as isotope-5, was discovered on the world of Makeb.