I admit, I tried to keep that one open ended for player interpretation. The traditional one is "cold iron" but that is still open to all sorts of meanings. (Smelting technique, iron content, etc.)

If you wanted a rough guideline, I would say that the more iron is in an alloy, the more likely it is to affect Fae. Aluminum wouldn't set them off too much. Steel could be very uncomfortable, depending on its content. I also imagine "direct contact" means skin to iron contact; clothing would at least dampen the effect.

Though please take the above as just a guideline, instead of an ironclad rule.