Create a clean basic certificate containing the jewellery item's stated specifications and observations. The document is intentionally transparent about what it does and does not establish.
The certificate can summarize the item's description, weight, purity, dimensions, reference number and stated test information. Keeping the underlying evidence or measurements available makes the certificate more useful.
A document generated by a website cannot independently prove that a jewellery item is genuine, that a purity claim is correct or that an issuer has legal certification authority. Those claims require appropriate evidence and, where relevant, independent testing.
Yes. Enter it as the issuer.
Yes.
No.