I'm always really skeptical about stones like these, I hear too many stories about water chemistry disasters after trying out rocks that weren't necessarily meant for aquariums.
Boiling or pouring hot water will only sterlize or kill the germs, but this will not change the ability of the stone to slowly release sediments or elements over time that might change the water chemistry.
Perfect I was about to create a thread just like this.
But more to ask about the type of unidentified rock in my box of unused tank ornaments.
I'm pretty sure loch is right. It's a type of gneiss. Granite doesn't have a glittery type surface like that.
I also agree with option's chemistry point. Hard to clean the structure of the rock itself. Bacteria/viruses are easy to eradicate but you need to boil for at LEAST 5 minutes. Pouring hot water on the surface of a rock doesn't do much.
Oil residues are HARD to clean.
Anyway, not to hijack, but what do you all think these rocks are? I'm most interested in the BROWN rusty rock. The person I got that one from was a aquascaper.