I just rewatched the Olenna and Sansa scene and you can clearly see when she takes one of the stones. I also heard the "plonk"-noise while she was walking back to her seat. However that could not have been the moment she dropped the stone into the jar, because it happened while she was before Margaery. The jar used by Tyrion later to fill Joffreys cup was standing before Cersei. Therefore I think she dropped the stone on one of the fruit plates next to Margaery. She took it a some point and put in the the filled cup after taking it from Joffrey just before he was cutting the pie. I think it was the horse of the WW during the last scene of Season 2.