"I am not capable of casting resurrection, much less miracle, of my own power. I could test that it is genuinely diamond, but only by breaking off enough of it to cast raise dead or grind to dust for restoration, both of which would significantly decrease the value of the gem. Your proposed test, with some amendments, would suffice, but with some of the same downsides; since actual surety is impossible to acheive, the goal of these kind of assurances is to make faking the matter more expensive than genuinely achieving it. When it comes to this kind of money, however, that is rather easier said than done; with enough gold to purchase wish diamonds on the line, even an archmage might be tempted, so I need to to hedge out both known spells and unknown magical capabilities.
"From my perspective, that leaves a few possibilities at different tradeoffs between secrecy, value to you, and time discounting. The first involves my confirming your story, either via your methods or by me communing with Abadar, purchasing them from you, and then selling them. This will not require that I tell anyone else where I obtained them or any significant additional work on your part, but it purchases them with time and reduced profits for you per diamond. The second proposal is that I do a sending to Archbanker Sevandivasen of Absalom or Archbanker Kalit Maron of Totra, and they arrive to negotiate with you. This would mean leaking some information about them going somewhere and involve informing them of your identity and of the fact that you have a significant number of diamonds, but they are almost categorically trustworthy in this sort of matter, would be able to purchase more diamonds from you, and it would be harder to link them liquidating diamonds back to you after they confirmed that the stones are genuine. This also has the potential to be significantly faster than other options. The third is that tomorrow I ask Abadar for a plane shift, head to Aktun with the diamonds you wish to sell, and have the axiomites confirm their veracity and then sell to them. Mortal goods, even diamonds, don't go as far in Axis as they do on Golarion, but this could allow for you to earn more money than I have available without anyone else on the planet ever needing to learn or waiting."