Black Metal Scrap (Dark Green/Black colour) from Fulings (and Fuling Village Chests) needs the Blast Furnace.

Scrap Iron (Brown colour) from the Sunken Crypts (and sometimes Swamp Creatures drops) needs the Smelter.

Iron Ore (Light Brown colour -- material like Copper/Tin Ore) is something I have only seen spawned from the -console/Devcommands. So if you ran across that somewhere on the Map and Pick-axed it, I have no idea how that gets processed. Mistlands, Ashlands and the Frozen North are not completed Biomes yet and it may come from there.