Mithral

'Mithral is a precious metal found in mountain ranges and the Underdark in certain areas of Faerun. Items made out of Mithral weigh half as much as that item's steel equivalent. Pieces of armour made mostly out of Mithril also allow the person wearing the armour to make better use of their natural agility and are less restrictive on the spellcasting of arcane spellcasters.