Hey Laurie Lou - sorry to hear this has happened.
A similar thing happened to me recently when I sold one of my dad's tennis rackets to someone via ebay. The buyer contacted me to say they had checked the racket out with a 'specialist' and they accused me of selling fakes. I think they were just trying it on and were hoping I would balk and give them a refund (and let them keep the racket). They threatened to leave my negative feedback if I didnt give them a refund within 3 days....
I was so mad and upset. I rang ebay helpline for advice, which I recommend you do too - they were quite sympathetic and I asked them to log on their computer that I had rung for advice and then made sure I followed everything they told me to do. I thought that would go in my favour because I wanted to demonstrate I was a concerned 'seller'
They advised me to contact the buyer by telephone - you can request the buyer's contact details through ebay.
I tried that, but the buyer wouldnt return any of my calls so I followed it up with an email. In my email, I said that I was sorry they felt the racket was fake and I had tried numerous times to contact them by phone. I also stated that I was 100% confident that the racket was genuine. I told them that I had the original receipt and if they wanted to take it further, they could file a 'significantly not as described' claim through ebay.
At no point did I offer to accept a return or give them a refund because to me, that would be like admitting the racket was fake - which it most certainly wasnt!!
It was quite stressful for a few days but in the end, the buyer disappeared and I havent heard from them since. They never left me negative feedback.
At the end of the day, it's up to you...... - do you stand your ground (you know it;s real and work in a jewellery store!!!) or do you just want to give a refund? However you do hear horror stories of sellers who accept returns, who end up receiving a fake bracelet and buyer keeps your real bracelet, your refund and disappears off the face of the earth!!! That's another reason why I made a point of not giving my buyer the choice of a refund or return.
Good luck - hope you get it sorted.