Buyer is calming jewellery is fake

By a diamond hallmark I meant that most modern jewellery containing diamonds has a mark next to the hallmark which states the points eg 0.50 pnts. I have asked the buyer to forward a copy of the jewellers report to me. But they tell me that they haven't had it in writing as it would cost £60 for a written statement., unless I wanted to cover the cost. Buyer says that the word of the jewellers is all that is required which I know is nonsense. I then asked which branch of jewellers and I got told it was a small independent one near to where the buyer lives. The buyer is in Dundee whereas I'm in Suffolk. I did make it clear in my online ad that the stones were natural diamonds that had been irrigated to enhance the colour and detract from inclusions which is how most coloured diamonds are treated. When the item was purchased all that time ago a diamond certificate wasn't given with it, I also made that clear in the ad. The receipt which the buyer has states the item code and diamond/ precious stone jewellery. I had the necklet cleaned and inspected where it was bought from just before I sold it and a store assistant said the stones were around M in colour and I4 in clarity with a fair cut. Natural mined diamonds yes but not high quality hence the purchase price. I also asked what a equivalent would cost at today's price and was told around £400 to £450 I made the buyer aware of this too. I have been in contact with paypal and they have said that the buyer requires some checkable proof that the item contains fake diamonds or other stone that is of lesser value to a diamond. Obviously the buyer wants something for nothing and thought paypal would just take their side.