As far as I'm concerned, customs declarations are for items that you are leaving behind after you leave; things like gifts, or items you intend on consuming, selling, in the country, or intend on coming back to use again. I'm assuming the ring will be leaving the country along with the two of you?

Thailand's customs is rumoured to be the country's most corrupt government department. I would recommend doing everything possible to avoid contact with any customs official. If I was involved with any customs problems at the airport, I'd surely want a third party, likley the police, observing.

I'll also ask the question that many readers are wondering....
... why would you bring a $30K ring to Thailand in the first place? You're not worried about it disapearing? For sure, I would NOT be putting it in any Hotel safe or Front desk safety box.