Customer wants a refund after 5 years

I run a small business, and was contacted by a customer who purchased an item of jewellery 5 years ago. They contacted me to say the necklace was dirty/mouldy and they wanted a refund. I kindly instructed them on a cleaning method, but they refused this suggestion and once again demanded a refund. I then offered them a replacement at a reduced cost, and they have also refused this offer and demanded a refund and have started to threaten me. I am really taken back by this situation, especially as...