I have some petroleum coke from a delayed coker unit from Woods River ConocoPhillips refinery which processes Canadian crude and tar sands crude. I was thinking about glueing to a tube then casting it in clear poly resin. Not sure if you'd want to cast it without the tube in it becasue it could catch fire since it's used as an adjunct to make coal burn hotter.
Being in the oil business and pen business, it ws about the closest thing I could come up with, myself. I have some PetCoke if you want some. It's a black carbon powder mixed with shiney black carbon balls in it.
Petroleum Coke is basically the residual heavy oils left over in the refining process and it is cooked in these giant furnaces until it carbonizes. You can also look at for the RhinoPlastic "coal mine" for a nice crude oil colored blank or maybe "mississippi mud" which is brown and black.
If you want some petcoke to cast, let me know and we will figure out how to get it to you.