[nebulousthree]

You shouldn't trust any old certificate more than it looking shiny. But if a third party that you recognise and trust happens to recognise the jewelry or the jeweler themselves, and goes so far as to issue a certificate attesting to that, that becomes another piece of evidence to consider in your decision to purchase.

[ethbr0]

Art and antiquities are the better analogy.

Anything without an iron-clad chain of provenance should be assumed to be stolen or forged.

Because the end product is unprovably authentic in all cases, unless a forger made a detectable error.