A chunk of pure coal. Can be used for fuelling the furnace and as an ingredient of various substances, like gunpowder . Used as a region when reducing metal ores to pure metals. Also used as a pigment for making darker hues of paint. In absence of mineral coal, you can burn wood in an oxygen=poor atmosphere in the furnace and obtain a similar substance: charcoal.
Coal can be crafted into solid coal block, which will last longer as a fuel in the furnace, versus coal chunks