Fairly easy, but tedious to do.
You'll use the Controled by variable option on the pulldown field. You'll need to create a list for the main base materials. Then a separate list for each of the base > stock materials. Then in the Controled by variable, you just set the logic: If select copper, then show the copper stock list.
This example uses Vehicle Type - Car or Truck.
that part I got down its somehow referencing the Stock Material selection to reference a part number the guys here know the materials but not the part numbers and I want that to automatically populate when they choose the material they want.