Diabase or Dolerite's a mafic, holocrystalline, intrusive igneous rock equivalent to volcanic basalt or plutonic gabbro. In North American usage the term diabase refers to the fresh rock, whilst elsewhere the term dolerite's used for the fresh rock and diabase refers to altered material.. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine grained to aphanitic chilled margins which may contain tachylite (dark mafic glass). Petrology Diabase normally has a fine,… (More on Diabase)