The Collaborative International Dictionary of English v.0.48:

Diabase \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.
   [1913 Webster]