Personally I'd do the following:
1) After receiving the feedback message the buyer via eBay messaging, ask him why he thinks the gaskets are not copper. If you have proof that they are copper (the original packaging as someone else suggested or doing a scratch/sandpaper test to show the underlying copper), attach that. Proof is key.
2) If they reply and acquiesce that they're wrong, politely ask them if they can change their feedback. Submit a feedback revision request.
3) If they refuse, contact eBay support, try to get someone on the phone who will look at the chat. The key is documentation, documentation, documentation. If you have proof, it will make it more likely that they'll help you out. It also helps to seem like the more "reasonable" one. People have an intuition for these things, if the buyer is being disrespectful, it'll make it more likely that support will take your side.
Your biggest problem is that you contacted them outside of official eBay channels. Never, ever do that. First, it opens you up to harassment claims, and second, eBay has no way of hearing/seeing what goes on outside of their system. They have no idea what you did or did not say to the buyer.
At the end of the day, some people are just bad people. I know the type, leaving bad feedback makes them feel some semblance of control in their sad, inconsequential lives. They wanted someone to redirect their anger and feeling of lack of control on, and you just happened to be the target. You can't please everyone, but you can cover yourself by making sure your listings are as accurate as possible, and your interactions with your buyers are as respectful and professional as possible.
