Nephrite is a rock, but largely consists of tremolite-actinolite amphibole minerals; while jadeite largely consists of a sodium rich pyroxene. Amphibole and pyroxene are related minerals. Amphiboles form in environments with access to aqueous fluids, while pyroxenes are anhydrous.
Both nephrite and jadeite are formed by the action of hot fluids within serpentinite. Nephrite is most commonly found in obduction zones -- areas where the oceanic crust (normally subducted) has been emplaced on to the continental crust.