I once gave a piece of coal to a family member as a Christmas present. I even wrapped it and put a bow on it. Let me tell you. Wrapping coal, if it's not in a box or bag can be a bit of a pain. Although, it's not like anyone really expects to receive a lump of coal as a gift so a shoddy wrapping job isn't going to give away the contents of the parcel. It's more about the sentiment and not the wrapping, right? It wasn't so much that this person deserved coal... it was just a chance for free hilarity. And, oh, we got it on video. Greatest Christmas ever. Well... for me. I don't think the recipiant was too happy.
My one piece of advice, if you plan on giving coal. After they open it and realize what it is. Duck!