Mithral

Known to the dwarves as truemetal, this shining silvery-blue metal is lighter than steel yet just as durable.
Equipment made of mithral weighs only half as much as the same item made of normal metals, and has no Strength requirement. Additionally, they count as one class lower. (Heavy counts and Medium, Medium counts as light)
Weapons without the Heavy Property gain the Finesse Property.
Additionally, light and medium armor made of mithral can be worn under normal clothes.
While wearing it, you can apply your full Dexterity modifier (instead of a maximum of +2) when determining your Armor Class.
In addition, this armor does not impose disadvantage on your Dexterity (Stealth) checks.
Small creatures do not have disadvantage on attack rolls with heavy weapons made of mithral.