Diabase

Diabase (also known as dolerite or microgabbro) is an igneous rock that is often blue in colour. It is rich in magnesium and iron.
Dikes and sills in igneous rock are often made of diabase. 
Diabase is mostly a silicate feldspar known as labradorite (about 40% to 70%). Most of the rest is pyroxene minerals (usually augite). Small amounts of hornblende, olivine, magnetite, and quartz are in some diabase rocks.