Relief is a term used to describe how prominent a mineral appears in a thin section in plain light.
Generally, a mineral which looks grayer in plane polarized light has higher relief. Both Granite and Diabase are Intrusive igneous rocks, which means they both cooled slowly deep in the crust. This is a 10X magnification picture of a Basalt, a common Volcanic igneous rock. Volcanic rocks form from Lavas which erupt at the surface and cool rapidly. The lava that erupted to make this basalt is of about the same composition as the magma that cooled to make the Diabase, so the crystallizing minerals are similar.