Metallum is the "metal" aspect, found in all metallic items. It is composed of Vitreus and Terra. (Before 4.2.2, it was Saxum and Ordo). It is a basic resource, key to a number of important crucible recipes (ore purification, nugget duplication, and voidmetal). Iron nuggets are a "perfect" source, but can't be used directly in the crucible, because they trigger the iron duplication recipe. More commonly, it will be distilled from mob drops (armor, swords, tools).