Mix some vinegar and lemon juice, pour on rock of it fizzes, bubbles and might go white don't use, if it just pours of rock and doesn't reacted its safe. Now boil some water and add rock for 15-30 mins add a cap or two full of bleach rinse of and let it cool should be fine to ise, make sure you give I think a scrub from dirt and stuff too. It would be cool to see some photos too
what peterfishkeepin said in general. vinegar works fine, if the stone is reactive that will tell you by fizzing and bubbling. if its neutral nothing will happen, and then it's likely safe, avoid any rocks that seem too sharp or too crumbly,
Then take care of bad bacteria that might be on the rock, A soak in Hydrogen peroxide will kill everything on it, and if it's porous whatever might be inside of it. You could boil it, it will be hot for a while.
you could heat it up many ways but if there is a void inside the rock it could crack, or even explode with heat, just keep in mind the possibility, like wet rocks around a campfire.
rocks can explode when heated.
I personally like just vinegar because it's adequate, and hydrogen peroxide because once it evaporates it's totally neutral. check for any signs of rust or heavy metals on the surface visually. Rocks are made of minerals, you don't want Iron or copper or something like that sitting on the surface of the stone.
What I do. wash it and clean it up of loose bits, then inspect it for any signs of problems, or sharpness that might cause an injury to a fish
if it passes that and doesn't crumble while washing,and doesn't have rust or anything that looks like a problem on it, then a vinegar test, and if it doesn't fizz up, then I soak in a container of hydrogen peroxide and water, 5 parts water to 1 part hydrogen peroxide, and let it sit for 30 minutes, then take it out and let it dry for a day and evaporate out of the rock.
I've never had an issue adding rocks and slate and stuff doing it this way,
but boiling I have had slate split and fracture, I just don't like the risk of heating them to sterilize them, I also don't like the bleach, just because chlorine in general is a no no, although given enough time, chlorine evaporates out also and offgasses.