Uru is a metal ore from the first moon in existence, and has existed since the beginning of the universe, being said to be rubble from the rock of creation and the strongest substance in all the realms. It is purportedly unique to Nidavellir, one of the Ten Worlds. It resembles stone, but it also appears to have metallic properties.
How do we know Thor’s hammer is made out of metal?
https://marvel.fandom.com/wiki/Uru