There's an old cartoon depicting a man standing outside of a Jeff Koons art show with a table full of inflatable animals and flowers for sale.

A sign says "your friends will never know the difference!"

Banksy did something similar where he sold art in New York for $60 without indicating they were his work. [1] After this was revealed, some of those works that were sold that day went for six figures at auctions.

[1] https://www.theguardian.com/artanddesign/video/2013/oct/14/b...

That's kind of an opposite, where they bought something they thought was authentic but not famous, but I take your point.

And that, ladies and gentlemen, is how you launder money.

I mean, it could be the case that the real diamond isn't there at all, and is back safely in some vault.