With Christmas on the horizon, many of us are thinking about gift giving. In this month's installment of Field Notes , Susan Knight discusses the virtues of coal (just in case you get some in your stocking). Christmas is coming. Are you worried about what you might find in your stocking? I am hoping for chocolate and gummy bears, but a nice lump of coal might not be too bad, either. Coal burning is now considered an environmental scourge. But the phenomenon of coal itself is fascinating and in