There are diamond rings and there are diamond rings. The immediate assumption that it will appear as an engagement ring shouldn't be made quite yet.
Can he afford such gifts? Or maybe it's one of the Kay Jewelers specials. I certainly wouldn't pull him aside before the party regardless of when.
Although it is customary in this country to open your gifts as part of the party, it's not etiquettely correct. The party is supposed to be just that and presents opened at home after. Should you open them at the party, a 'thank you' after will suffice(don't forget those thank-you cards later!
and you can always return it to him later (you might get other things that make you shudder as well-can't disapprove right then and there with those either).
Just have a good time and worry about reactions, etc., later.
Edit because I didn't see the date of the OP: I hope your worry was for naught. How'd it go?