It does seem difficult to believe that the Saudi's managed to acquire an enormous blue diamond, 50% larger than the largest confirmed to exist, in complete secrecy without a single peep of it leaking out by 1989.

But on the other hand, if it was entirely made up, why would they invest the resources and diplomatic capital over so many years in hounding the Thais? Over some ordinary jewellery easily replaceable at any high-end jeweller?

I can't see how they would gain anything from it. And Thailand doesn't have any real enemies except for maybe Vietnam, so there's no credible political angle either.