Olenna passed the gem to Margaery at some point when everyone was busy moving around and chatting after she snatched it off Sansa's necklace. Then Margaery dropped it into the wine right after Joffrey passed his goblet to her to set down on their table so he could chop open the pie with his new sword. Joff takes a bite of the birdshit pie and then orders Tyrion to serve him the wine that is now spiked with the poison so he could wash it down.
Lol, btw. Cersei was hilarious in this episode. Just the most evil bitch but it made me laugh.
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