You are using an out of date browser. It may not display this or other websites correctly. You should upgrade or use an alternative browser.
Recent content by Patten
The friendliest place on the web for anyone with an interest in aquariums or fish keeping!
If you have answers, please help by responding to the unanswered posts.
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).