While I’m willing to think about the black diamond and PP/Tink, I don’t think Snow randomly came across it.
The diamond always felt very odd to me. There was never any indication in the past that Regina had this fail safe tucked away in case of emergency. Remember, she though she won. She had her victory, so what’s with the backup plan? She isn’t that kind of woman. She’s rash and hotheaded and when she cast that Curse she honestly thought she’d won forever. The diamond felt like a last minute hail-Mary type of writing from the writers who were like “we need to move Regina closer to redemption and find a way to demonstrate Emma’s powers and also get Henry taken by GOAT. I know…black diamond fail safe.” It never really fit with anything we knew about the Curse or Regina. My point is I’m not sure we are ever going to hear about that diamond ever again. It’s purpose has been served and I don’t know if there is a larger story to be told.
With that said, I do think Tink may have been making deals with Regina at the behest of PP. I just don’t know what for. Maybe the diamond, but that would have to be really close to the casting of the Curse. And what does PP get in return?