Mithral

Mithral, also known as mithril, was a precious metal produced from ore found in mountain ranges in certain areas of Faerûn and in the Underdark.

Description
Mithral was produced through a process similar to that of steel-smelting. Items cast from mithral weighed only half as much as similar items forged from steel. Since pieces of armor made from this valuable metal were very light, they allowed the wearer to make better use of their natural agility and were less restrictive on spellcasting.

Rumors and legends
Among some bardic circles it was said that mithral combined with steel could create adamantine, but according to dwarves, this was laughable.