Copper is a chemical element with symbol Cu and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orange color. It's is also used along with zinc in the production of the metal alloy brass.
During the american civil war, some confederate soldiers wore a copper penny around their necks for luck, it's unknown if that's true and/or how much luck this penny would bring.
The only known way to definitively kill a nachzehrer is to put a copper coin in his mouth when decapitating him. In the episode Baby, Sam Winchester states that the coin must be minted pre-1982, since the ones minted pre-1982 are 95% copper, while the ones minted after 1982 are only copper-plated zinc. It's unclear how much copper exactly the coin must be made of for it to work, but it is at least 95%.
Creatures Affected by Copper
- Nachzehrers - Placing a copper coin minted pre-1982 (that is, at least 95% copper) in a nachzehrer's mouth combined with decapitation is the only thing that can kill it.
- Supernatural: Mythmaker (Flare's Copper Circlet)