Tungsten carbide and uranium cement have been postulated. https://en.wikipedia.org/wiki/Space_weapon#Orbital_bombardme... Interestingly, these are not prohibited by any treaties. The US Air Force estimated a 1-foot (.3 meter) diameter tungsten pole would impact ~Mach 10. It would weight 9 tons and have the same destructive force as 7.5 tons of dynamite, but it would only take 15 minutes to get anywhere in the world and there would be no ICBM launch warning. https://en.wikipedia.org/wiki/Kinetic_bombardment#Project_Th...

If I remember rightly (it's been a while since I read up on them), they proposed using tungsten rods initially. Not sure if they'd still go down that route, or whether they'd use something a bit more exotic (carbon nanotubes is always the go to answer for exotic-scifi style things).