What 'rocks/minerals'' are safe for Aquariums? I want to engrave an Ancient Greek inscription for tank (Interesting thread!)

[deranged chipmunk]

slate is fine to use. i have used italian slate from a defunct billiard table in my aquariums before with no issues. not sure where you would find it in your country though. limestone is a no no. it will release minerals into the water, making it very alkaline and hard. like carlos mentioned, marble and granite are ok. i would also stay away from sand stone. you can do a quick test on any rock for it's "usability". a couple drops of vinegar on the rock will do. if it fizzes and bubbles, don't use it