Its more of a cinderblock that broke off. How long would it take for it to dissolve and produce too many chemicals, possible turning the water to acidic?
just go to the nearest river or stream and you should be able to find plenty of nice rocks to use in your tanks. as for the acid test, I've heard you can also use vinegar. if it boils on the rock then the rock will raise the ph.
I got a bunch of rocks from the stream outside my best friend's house. like the others said, scrub them extremely well. I used a wire grill brush and it worked great. also if you don't want to boil then you can also bake them. I baked the rocks in the oven at 400 for one hour. insert a toothpick in the middle and if it comes out crumbly then they're done.
Heres what id do, if i see something i want in my tank, soak it in salt water for a couple weeks, change water a couple time, salt helps to pull out toxins, then throw it in a bucket of fresh water for a week, then test the ph
i also have asanitizing function on my dishwasher that runs for like two hours and it works awesome just make sure to run it empty once first tio get out all unwanted nastys