Haha @POTRZBE it's a gem, for sure.
Is real coal too dirty for our purposes? I imagine I would need to remove my fines and maybe I would be good. The coal slag dust is minimal so far.
I have some black sand on the way. I considered activated charcoal but I thought it might break down over time. There are a few Etsy shops that appear to be selling aquarium gravel that looks like what I need but my concern is sourcing it twice. I worry they only sell it because they picked up at a garage sale or something. So I tried the sand. We'll see how it works! I'll report back.