In popular scientific literature and science fiction, though rarely in formal scientific literature, the term "neutronium" is often used for the extremely dense matter at the cores of neutron stars.
So-called "conventional" neutronium, also known as degenerate matter or "element zero" on the periodic chart, is theorized to consist of nothing but neutrons, with no protons in the atomic nuclei (and thus no electron shell, which prevents it from interacting chemically with other elements or forming molecular bonds with them). Thus, it is commonly hypothesized, something made of neutronium would be virtually indestructible. In practice, however, since neutronium would require immense ongoing pressure to retain its state, it would almost certainly disperse explosively outside of its natural environment.
According to Cracken's Threat Dossier, neutronium is a naturally occurring metal that can be found in mineral veins on moons.