Iron is a native resource, one mined from bodies rather than made, and the largest share of the material in every tier. A Smelter makes it into iron plate, where nearly every chain starts, into steel plate directly with carbon and oxygen, and into slugs. Raw iron also builds the Harvester, the Outpost, the Workshop, the Generator and the Smelter itself.
Iron is dense, so in a body hot enough to melt it, it sinks to the core.