There is a type of Coal called Parrot Coal (goodness knows why, I thought they used Canaries in the mines) it's very dense and carves really finely. I have a small boot which is made out of the stuff. I have see quite large pieces including a small pedestal table carved from it. Do a search for Parrot Coal in Google images to see some examples.
The comparison of the weight of coal and the item was mentioned, but I don't see any mention I the original post about weight or size of the item, perhaps these could be supplied...
Good luck with your quest.
Mike