An exotic substance found only from neutron star ejecta and other such exotic sources, neutronium ore is refined into Neutronium Fuel, which is used as starship fuel by some civilizations. It is far too unstable for use in building ship components.
Neutronium is a colloquial and often misused term for an extremely dense phase of matter that occurs in the intense pressure found in the core of neutron stars. When a massive star creates an iron core whose mass exceeds the Ch limit, it will collapse and create a type II supernova. The neutrino is emitted from the core, leaving the neutron behind. The material that remains has a density of approximately 1014-1015 grams per cubic centimeter. A teaspoon full of this matter would have a mass of 100 million metric tons. This material has often been termed neutronium.