Validium is a living metal created by the Time Lords, the ruling elite of the planet Gallifrey. Both time active and capable of processing vast amounts of energy, validium was created as the ultimate weapon of the Time Lords--though once it had been completed, the Time Lords realized that their weapon was both self aware and possessed of a malign destructive intelligence. Subsequently, the Nemesis Device was sealed away, and any knowledge of it struck from the records of the Time Lords--and would have remained hidden for all time if not for the intervention of a certain Time Lord.
Its participation in Suburban Senshi history began when Dr. Xadium utilized fragments from Nemesis to construct a new body for the artificial intelligence rei.bot. Ignis is made of a proto-validium metal that is not as strong, and lacks time travel abilities, but has trans-dimensional properties.