That's really nice! Is that rock or a piece of metal lol... Not sure you can easily get it appraised here, maybe bring it to a rock specialist?
Well the vinegar test is for calcite. It doesn't show if it's safe or not, just if it will harden you're water and raise the pH.It's a rock, ill probably give it the ol' vinegar test after work
Well the vinegar test is for calcite. It doesn't show if it's safe or not, just if it will harden you're water and raise the pH.
That appears to be a piece of obsidian and/or slag. Either of those would be completely safe to put in the tank, as they are both effectively glass(that's exactly what slag is).