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. This texture, consisting of randomly oriented, small grains, is called Felty , because it sorta looks like felt. If you have problems, refer to the higher power pictures below. The black grains visible in the 50x plain light image of Basalt are opaque oxide grains, probably Magnetite. All of the above rocks display different varieties of Crystalline textures , which is to say that these rocks formed via the growth of crystals.