Coal was one of the gift items during Holiday 1997.
Coal ingots were created by smelting the Coal from the 1997 "Naughty" Christmas Bag. Coal ingots are a slightly different shade of color than the Shadow ingots. Shadow ingots have a slightly blueish tint, whereas the Coal ingots do not.
Some players were able to collect enough Coal ingots to create sets of armor now commonly referred to as Coal Armor. GM marked armor made from Coal ingots can be quite valuable.