I do have a hard time with hardness and pH, our tap water comes from the lake and both specs tend to fluctuate, so I try to keep rocks that won't affect pH, unfortunately a few of the rocks I have I don't know what they are. I think I have some Texas Holey rock which does raise pH and calcium.
You want hard insoluble rocks. Slate, sandstone, granite,petrified wood, quartz are some that won't affect pH/hardness. One way you can tell is by the vinegar test, put a few drops of vinegar on the rock, if it fizzes than it will affect pH/hardness, though that does not work for all rocks, there are some other things you can use, or you can just let your rocks soak a bucket of water and test it.
And that is about all I know about rocks. Thanks for the comment!