Quicksilver

Quicksilver's more accurate alchemical name of "mercury" comes from the madness induced by inhaling the toxic vapours, whereas the more common name just comes from this metallic substance being a silver liquid at room temperature. Quicksilver is extremely toxic and must not be handled without some sort of protection, but it is also quite useful for any alchemist as trace amounts of it are used in many particularly powerful concoctions. It can be purchased from Alchemy shops or derived from cinnabar ore.