Necronium was a proposed metal that would have been used in a Mining and Smithing rework mentioned by Mod Jack at RuneFest 2013. However the planned rework was not completed and necronium is no longer planned to be added to the game. Smithing necronium would have required levels in the 70s, bringing it on par with Barrows armour.
Mod Jack posted the following explanation for the lore of necronium on the RuneScape Official Forums in November 2013:
|“|| THIS IS CURRENTLY UNOFFICIAL AND SUBJECT TO CHANGE.
Thought you might be interested in this full text about the new metal Necronium from the Mining & Smithing documentation.
In the Third Age, when the conflict between the gods raged across the world unchecked for centuries, mortal casualties were beyond measure, though they must have counted in the hundreds of millions. Demons and other great monsters heaped bodies upon bodies in great mountains of the dead. The bones and blood and metal panoply of those ancient battlefields sank beneath the soil. Over the centuries they coagulated in darkness and steeped in the residual magical energies. When the people of the Sixth Age unwittingly disturbed these open air charnel houses they found new materials of remarkable strength.
Necrolith is a dark fusion of bone, third age metal and ancient magicks. The skulls of humans and larger, monstrous creatures protrude from the black, obsidian-like surface of this ore. In its presence the skin crawls. Thanous is a pale, greenish-white, powdery rock formed primarily of bone which burns with a brilliant green flame and a sound of distant screaming. When necrolith is heated in the flame of thanous it does not melt or sag but rather stretches and rises as if straining for something. It forms itself into the plates and shapes of armour and weapons, though bizarre and misshapen. A skilled smith can guide and correct this process and create usable and extremely powerful equipment. The resulting necronium has a dark blue sheen and is still covered in the skulls and bones of the dead that grant it strength.
— Mod Jack