smelting coal ore

You could smelt coal ore into diamonds in the blast furnace, it just takes a lot longer than smelting anything else in a regular furnace, taking 1 1/16 coal per coal ore (so, to make 8 diamonds, you would need 9 coal, 8 coal ore [for which you need silk touch], and a blast furnace, it will not work in a regular furnace, it will just give you coal)