Mercari complaint: Honest Seller who was made to Feel like a Criminal

I sold a 10k Gold Rope Chain necklace – Hollow 3mm to a buyer. The buyer then wanted a refund claiming they were worried about the authenticity. They claimed to have taken it to a pawn shop to have it verified as 14k filled, not 10k as I listed. The problem is, they offered NO Proof of taking it anywhere to verify; they took NO pictures of any verification paper from the pawn shop.
I know for a fact the necklace is real 10k gold because I am the one who bought the necklace for myself from Zales jewelry store some years ago.
Within 2 minutes of me stating my case to Mercari, they had made their decision and claimed that I did not send the item as I listed it. This is a complete lie because I have the listing and the pictures. The buyer and I had talked several times about the necklace and I told them that I no longer had any paperwork on the necklace, but that there were even tests that can be done to see if a jewelry item is real or fake; I even had pictures of one of the tests I had done shown on the listing.

Mercari said they would investigate both sides of the claim, but I don’t see how that’s possible to do in 2 minutes??
What really worries me is that there have been a number of scammers who buy things from sellers, then ask for a refund, they send a fake back to the seller, and then get their money back also!
I am so worried that I will now get back a fake necklace instead of the real 10k gold necklace I sent the buyer.
Please, I need help. Not only did Mercari make me feel like a criminal when I was totally honest about everything (the item I sent was exactly as I described), but now I feel like I am going to get ripped off and not get my real necklace back!