Assuming the pendant is small and delicate how about a CARD TRICK?
Force the FOUR of diamonds on the recipient (who as a prize has won a chance to appear with you on stage) and then produce the FIVE of diamonds from somewhere which would be impressive if it were the correct card.
Correct your mistake by folding the card, tearing the center pip from it and handing her the mutilated card.
Now, as an afterthought say "You might as well have this too" and transform the torn pip into the pendant using a TT.
A bit of wax on the inside of the tip will cause the pip to remain inside as you pour the gift from the tip. You could also simply have the necklace in a loose fingerpalm under the tip.
Other methods are of course possible...