Iron
Up until the discovery of pulsars it was thought that no atomic nucleus can survive the gravitational forces at the core of collapsed stars. It was thought that the remnants of those collapsed cores consist of some electrons, some protons but predominantly neutrons (thus the name neutron stars). However the discovery of radio waves in pulsars means that neutron stars contain rotating iron cores! This means that the iron survived! Today we know that iron has the strongest bound nucleus among all elements.
[Quran 57.25] ... and We sent down Iron, in it great might...
Iron and most of the elements on Earth came from outer space. Iron was known and used long before the Quran however it was not the best choice for weapons because it rusted and it easily bent. The miracle is that the Quran said "in it great might" at a time when they only knew that it rusted and bent. Today we know that iron has the strongest bound nucleus among all elements. Learn more.