Neutronium is an extremely durable and rare material used for top-tier armor. It is not obtained through mining, instead from using the SolarScoop on a Neutron star.
Description
"The most durable obtainable material in the game. It is a strange form of matter found from the crusts of neutron stars. Due to the nature of neutron stars, neutronium is the hardest known material as well as the densest known material in the universe."