Many vintage jewelry metals are hard to determine when not marked. I can only narrow it down for you by the magnetic quality of it. Here are the magnetic qualities in different metals to help you determine if this item will work for you:
Magnetic - Cobalt, iron, nickel, neodymium, samarium, and gadolinium
Slightly Magnetic - Rhodium (nickel free & hypoallergenic)
Non-magnetic - Gold, silver, brass, copper, zinc, aluminum, pewter