I happened to notice some black poop in the coop the other day and was like "holy crap look at that". It was nice and firm with a white cap just black as coal. Started to notice more in the run and around their penned area. Well last week I had burned some brush and left the ash and bits there. You can see where this is going. Well they ladies decided to eat the black charcoal parts. While I was concerned but didn't see any adverse problems from this. Laying, eating and normal behavior are up to par. So after a little research, charcoal feeding was a regular dietary practice in poultry as well as cattle. But the practice has subsided. From what I read it had many benefits. So basically don't be concerned if your chickens dive into the charcoal but suggest don't let them near briquettes, don't care to use them anyway. I like hardwood lump charcoal myself.