Di"a*base (?), n. [F. diabase, fr. Gr. &?; a crossing or passing over, fr. &?;; &?; + &?; to go; -- so called by Brongniart, because it passes over to diorite.] (Min.) A basic, dark-colored, holocrystalline, igneous rock, consisting essentially of a triclinic feldspar and pyroxene with magnetic iron; -- often limited to rocks pretertiary in age. It includes part of what was early called greenstone.