dolerite

Igneous rock formed below the Earth's surface, a form of basalt, containing relatively little silica (mafic in composition).

Dolerite is a medium-grained (hypabyssal) basalt and forms in shallow intrusions, such as dykes, which cut across the rock strata, and sills, which push between beds of sedimentary rock. When exposed at the surface, dolerite weathers into spherical lumps.

In the UK the term ‘diabase’ is used as an alternative to dolerite, often implying an altered dolerite, as distinct from fresh rock.