Dolerite rock crystals. Light micrograph of a section through of sample of dolerite, mostly comprising large crystals in a finer-grained rock matrix. The colours seen, and the physical appearance of the rock, depend on the exact mineral content of the rock and how the rock was formed. Dolerite is a coarse-grained basaltic rock, meaning it is similar in composition to the basalts. Like basalt it is an igneous rock produced by volcanoes, and it is also a very common rock.