Nephrite is, in fact, a mineral species, not a rock type, it is an asbestiform variety of Ca-rich amphibole. The solid solution series (Mg-Fe) in question is tremolite-actinolite. ‘Tremolite fels’ is used to describe a tremolite-rich rock. The character of the tremolite in these axes probably makes nephrite an acceptable term. The semi-precious 'stone' jade is, mineralogically, either nephrite or jadeite (a type of pyroxene).