Training in the techniques used to purify ore and combine them into powerful metals. You know how to forge particular blends for an appropriate result (heat-resistant, bendable, conductive, etc.) It's your job to get the metal used by many other skills (like advanced smithing.) You can shape steel through various processes (casting, forging, sintering, etc.) Combined with Earth Sciences or Masonry, you can analyze and mine for elements specific to the area. You also know how to sift, purify, and appraise valuable gems and other precious materials.
At the highest levels, this should include the blending of plastics and special fibres with metals. This skill wasn't very advanced in ancient times and it may be best to roll it into Masonry.