We have a rock testing kit (I'm kinda a rock hound lol) and we have a very diluted solution of HCl, none bubbled when tested so I know I'm good there.
I'm doing an african cichlid tank and although my pH is only 7.6 they like 8.2 -8.4 pH so I could use a little raising, hence why I also looked for chunks of broken marble.
I know slate and quartize is inert in water thats why I went looking for it. I just got a few pieces that I question.
I know not to boil rocks but I didn't want to bleach or use vinegar on them and I had to sterilize. Every thing went fine, even if they did explode I was ready. The grill wasn't near the house, I had eye protection, I never left it unattended and I have a spare bucket of water just in case. I have a bunch of geologist friends and trust me, they wouldn't let me do anything that would cause me serious harm.
Infact I have one coming over to inspect the rocks/tiles that have survived until now (I've been throwing out the ones that flake of became softer in water) just to be sure.