I had happen to have lost a piece of jewelry in the theater today. Any chance that that would be something put to the side or to a lost and found center, or would it be thrown in the trash?

I lost a phone there once... When i returned a few hours later, they had it for me. Someone had returned it at the ticket counter

Talk to the ticket taker or an usher. They should know who to direct you to. If it was visible to the cleaning people, there's a good chance it was picked up and put in a lost items box. What you're talking about happens regularly in public spaces like a theater. Enough so that I'm sure there's a procedure that they follow.

Service desk will help but try s manager in the theater

They returned my phone also.