Advice on aquarium rocks

I have some really nice chunks of flat rock that I collected as kid living a few miles from Mt. Rushmore. It looks like dark grey granite with mica flecks in it. I've been saving them for years and I'd love to put them in my aquarium. Can anyone tell me the best way to check whether this rock is likely to raise my aquarium pH? I would hesitate to put it in the tank... the last time I did that with a different rock the pH skyrocketed and my GBR gilled like a madman for a couple days. I removed the rock and eventually got the pH back down, although it still hasn't returned to its original level. My water's pretty hard already so without a RO unit I don't feel like I can do a good test at home. Any advice would be helpful. Also, any recommendations on other rock that would be pH neutral would be greatly appreciated. Thanks!

petrified wood,and regular granite are good.alot of slate will work out.not sure about the mica.you could always put some in a bucket for a week and see what the PH changes too and judge if your water changing regement would suffice in keeping the PH at a acceptable level.

a simple test is to pour a little vinegar on it. If it "fizzes", it's a no go. This won't give you 100 percent safety, but will let you know if it is going to have a drastic effect.

I'll try both methods. Thanks.