Mithral

Mithral is a precious metal found in mountain ranges and the Underdark in certain areas of Faerûn. Items made out of mithral weigh half as much as that item's steel equivalent but exactly the same process is used to smelt it. Pieces of armor made mostly out of mithral also allow the person wearing the armor to make better use of their natural agility and are less restrictive on the spellcasting of arcane spellcasters.