Diabase is a mafic, holocrystalline, igneous rock equivalent to volcanic basalt or plutonic gabbro. Diabase normally has a fine, but visible texture of euhedral lath shaped plagioclase crystals set in a finer matrix of pyroxene, typically augite, with minor olivine and magnetite. Accesory and alteration minerals include hornblende, biotite, apatite, pyrrhotite, chalcopyrite, serpentine, chlorite, and calcite. The texture is termed ophitic and is typical of diabases. The feldspar is toward the calcic end, most commonly labradorite. Diabase is usually found in smaller relatively shallow intrusive bodies such as dikes and sills. Diabase dikes occur in regions of crustal extension and often occur in dike swarms of hundreds of individual dikes or sills radiating from a single volcanic center.

The Palisades of the Hudson River, near New York City, are an example of a diabase sill. The dike complexes of the Hebridean Tertiary volcanic province which includes Skye, Rum, Mull, and Arran of western Scotland, the Slieve Gullion region of Ireland, and extends across nothern England contains many examples of diabase dike swarms. Diabase is also called dolerite in many older references.