In my line of work, we use the term "engineered material" for things that have been selectively modified to outperform their naturally-occurring counterparts. So for me and my colleagues:
Iron is considered a naturally-occurring material, even though it must be mined, smelted, and refined into a useful form. Cold-rolled steel (ASTM 1008), however, is an engineered material, because it has been chemically and physically modified with additives at very specific temperatures to have very specific properties (corrosion resistance, hardness, tensile strength, etc.)
I don't know if it helps this particular argument, but that's how my brain has been trained. "Cold iron," in D&D terms, is iron that has been magically engineered to have specific anti-fey, anti-fiend properties.