So, I've started using a little "resource saving" strategy when I'm mining: if I hit an area underground that's made of dirt, I switch to using my hands and burrow out all the dirt. I usually stand a good chance of finding coal, iron, and/or redstone in such areas, but primarily coal. And some of the underground dirt blobs are very large so you end up hollowing out quite a spacious area. Since it doesn't require any tools to do this (doing it with your hands is reasonably quick) it saves me consuming pickaxes.