Quantonium is a powerful radioactive energy. It is the same energy that effected the human named Susan Murphy by growing her to the size of a giant. It might be a liquid mineral mined on Gallaxhar's planet. As it is revealed in the movie that Gallaxhar destroyed his own planet, this likely means that the known supplies of quantonium have been exhausted. As demonstrated on "Ginormicat!", its effect on cats is that they grow until they're bigger than Earth.
Blue liquid contained inside a black rock or iron.