"All of creation cares not whether you live or die."
Adamantine is a synthesis material elements which can be collected throughout the darksiders universe as you travel the broken Earth. Through its acquisition and turn over to Ulthane, he will smelt it down and use it to augment Fury's arsenal.
Fury can find fragments of Adamantine while on her journey to apprehend the Seven Deadly Sins. They are usually found sequestered across the battlefield of Earth as well as hiding in plain sight while on her quest.
Adamantine's main uses is as a forging substance to fashion as well as reinforce the weaponry forged by the armies of Heaven and Hell. It is often used by the other elder races for different purposes beyond strengthening warfare, like the fabrication or enchantment of a great many various relics and magic items utilized in story by their protagonists. In fury's case, she can turn over collected pieces of which to the Maker of Haven and they will embolden her Nephilim crafted weapon; Scorn for her.