Is it possible Emma and Hook have gone to get Dorothy? If the group has figured out that maybe Dorothy is able to help destroy Zelena they might have sent Emma to find her.
As for the necklace, I think its a flashback. Glinda said previously that she gave Zelena the necklace to store her magic. So if the sneak peek with DG is when Glinda and Zelena just became friends maybe the necklace is clear because Zelena hasn’t stored her magic in it yet. Her magic always seems to be green ie. with the cursed potion and when she disappears and what not.
I totally misread what you guys were saying about the necklace and Zelena’s skin.
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