Mithral

Mithral is a precious metal produced from ores found in mountain ranges in certain areas of Faerûn and in the Underdark. Mithral is produced with a process similar to that of steel smelting. Items made out of mithral weight half as much as similar items made of steel Pieces of armor made mostly out of mithral are very light, therefore they allow the wearer to make better use of their natural agility and are less restrictive on the spellcasting.

Folktales
Among some bardic circles it is said that mithral combined with steel can create adamantine, but according to dwarves, this is laughable.