Generally speaking I bring home like 50 billion rocks from my desert trips, annoys my dad.
Usually for me it's granite, slate or shales, gneiss, sandstones, lava chunks, quartzites, agates etc, anything pretty looking with lots of crystal formations. On my last trip I even found some really nice multi color jasper and chalcedony or calcite pieces, one was a green clear one that was near gem quality, but I lost it in the jeep I think. Just collect them from an area that isn't in a protected wilderness or where the gubmint is watching you or something and you should be fine.
My water is liquid rocks (plus cichlids and livebearers anyway) too so I don't worry about the hardness changing too much. Never had an issue with anything. Just be careful. You can pour boiling water on them a scrub them with a toothbrush to get any gunk off of them. But full on boiling them can make them explode.
I agree, don't boil them, or bake them. Any trapped water or moisture can turn to pressurized steam, then youve got a rock bomb just waiting to go off
