Mining economics often treats mineral extraction as if it already explains industrial material supply. This commentary argues that the assumption is too narrow. Iron ore does not become steel simply because it has been mined. It becomes steel through an industrial transformation system involving mills, furnaces, energy inputs, production technologies, emissions, efficiency constraints, and policy choices. Drawing on Benini, Enstad, Mersha, and Rossini's plant-level analysis of technical and environmental efficiency in the global steel sector, the article develops the concept of the industrial transformation boundary in mining economics. The source paper studies steel mills across 50 countries and estimates both technical efficiency and the shadow price of CO₂e emissions using a Stochastic Directional Distance Function. The results suggest that steel producers face different combinations of efficiency gaps and marginal abatement costs depending on technology, production pathway, and regional context. The article argues that mineral economics should not stop at extraction when the relevant policy object is an industrial material. Mines produce inputs, but industrial materials emerge through transformation systems that combine energy, capital, labor, process technology, emissions management, and industrial policy. The mill is therefore not a downstream detail; it is a critical economic boundary. The commentary does not claim that the source paper is a mining-extraction study, that all steel production pathways are equivalent, that electric arc furnace steel is automatically clean, or that steel-sector findings can be generalized to all minerals. Instead, it argues that industrial transformation deserves greater attention within mining economics because material supply depends not only on what can be extracted, but also on what can be made.