The vinegar test can be useful, but a lot of rocks/minerals that will affect hardness won't react to it. If something does fizz, you can be certain it will affect your water chemistry, but no reaction doesn't necessarily mean it's safe/inert.
philemon716, your rocks look to be granite schist. I am not sure if they will change your ph but they do look good.
I wasn't paying attention to the sparklies, so schist might be more accurate, it's been a while since I've done anything remotely geologic (and I was never good at the mineral ID to begin with...)
- BTW, what is the rule to prevent kitchen xplosions when it comes to BOILING??
I'm not sure that boiling is really necessary, I doubt that there is much risk of pathogens. Then again, it might be useful to remove any potential residues that may have accumulated on the rock wherever/whatever it was doing before.
I don't think there is any serious risk of explosion from boiling a rock. At most I can see cracking from thermal expansion.
Best I can figure, this started as a general warning about baking rocks (similar to being careful what you put around a campfire) as some rocks (typically sedimentary) can absorb water, and when heated rapidly, this can convert to steam and fracture the rock violently. I think I seen a lot of similar concerns/warnings over at Dendroboards, so I think it was just an initial precaution that got exaggerated with each retelling.
If I had a good reason to boil a rock, I wouldn't even hesitate, just find a pot big enough, and fill it with water, and fire up the stove.