Personally I use lava rock cause it's easy to stack to create a lot of caves and hiding spots. Slate is another popular option because of it's stacking ability too.
I'm trying to keep the rocks the same and right now I have a type of sandstone or slate with quartz in it and it looks nice with the black sand, I have one other small rock and a piece of bogwood and I put some more plants in since I got more fish today