Interesting...so trying to find any pens made fom coal, it seems from the less than 5 I saw, it needs to be crushed up and then cast...
That's possible, it may get too brittle otherwise. Would be worth a try if you could get hold of some anthracite. But you'd probably need a masonry saw to cut it, and it would probably be a tad hard on the carbide cutters. I wouldn't think it would be to tough to cast, there really isn't anything there to trap air.
Did they say how they were casting it Marc? I would think you could probably mix it in your resin and simply pour it, or maybe even mix it thick enough to pack it into the mold. Pouring resin over it, you're probably going to float it.
I wonder if it will drill without cracking and chipping after stabilization. I tried drilling a piece that I have, it just chipped and crumbled.
Extremely sharp bit required and go slow from what I was reading.